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A device to correct the effect of arc wander
Spcctrochimica
Acta,
1963,
Vol.
19, pp. 1531
to 1540.
l’ergamon
Press
Ltd.
Printed inNorthern Ireland
A device to correct the effect of arc wander* S. S. BERMAN, D. S. RUSSELL and L. L. T. BRADLEY? Division of Applied Chemistry, National Research Council, Ottawa, Canada (Received
28 September
1962)
Abstract-A compact device has been developed for maint)aining a beam of light emitted from an arc discharge incident on the spectrograph slit. A photosensitive detecting system monitors the The latter repositions beam and operates a servo-mechanism which rotates a refract,or plate. t,he beam should it begin to wander off the slit. Data are presented which show a significant increase in precision when this device is used.
INTRODUCTION ONE of the drawbacks to the use of the d.c. arc and a major contributor to erratic results in emission spectroscopy is arc wandering. Many proposals such as rotating magnets [I], gas jets [Z], special electrodes [3, 41 and rotating electrodes [5] have been made in an attempt to stabilize t’he arc and thus reduce wander, but few have dealt with the actual correction of the effect of wander in cases where it does exist. The most common method for dealing with the problem is by tilling. That is, the spectrographer watches the image of the arc on the spectrograph slit and moves the electrode system laterally in order to keep the image on the slit as the arc wanders. Most of the elaborate arc-spark stands are equipped with a tiller for this purpose. ARRAK and MITTLEDORF [6] cite and describe two optical systems to correct the effects of arc wander. One, the Linsenraster [7, 81, images the source with a mosaic of lenses in such a fashion that as the arc wanders the image moves to a much smaller degree. The second, suggested by LINDER [9], utilizes a biprism which produces side by side images of the arc. As one image moves off the slit the other moves onto it. Both systems result in a considerable loss in light intensity. CAVE [lo] constructed an electronically operated electromagnet to control arc wander. The device developed by the authors is a compact system mounted on the * Presented at the Ninth Ottawa Symposium on Applied Spectroscopy, Ottawa, Canada., 18 September 1962. Issued as N.R.C. No. 7457. t Present address: Division of Pure Physics, National Research Council, Ottawa, Canada. [l] [2] [3] [4] [5] [6] [7] [8] [9] [IO]
A. W. C. M. E. A. E. E. W. G.
T. MEYERS and B. C. BRUNSTETTER, Anal. Chem. 19, 71 (1947). H. CHAMP, spez Speaker 3, 1 (1962). R. JEPPESEN, E. J. EASTMAND and H. G. LOGAN, J. Opt.Sot.Am.. 34, 313 (1944). F. HASLER and C. E. HARVEY, Ind. Eng. Chem., Anal. Ed. 15, 102 (1949). K. JAYCOX and A. E. RUEHLE, Proc. 7th Summer Conf. Spectrosc. Its Appl. 1949, p. 10. ARRAK and A. J. MITTLEDORF, Appl. Spectrosc. 13,85 (1959). PKEUSS, Heidelberger Be&r. Mineral. u. Petrog. 4, 163 (1954). PREUSS, Spectrochim. Acta 11,457 (1956). J. LINDER. Unpublished work. (See Ref. 6). C. B. CAVE, Anal. Cllem. 20, 817 (1948).
1531
1532
S. S.
BERMAN,D. S. RUSSELLand L. L. T.
BRADLEY
optical bar which uses a pair of photo conductive resistors to monitor the position of the beam. A servo mechanism rotates a quartz refractor plate to keep the beam incident upon the slit. PRINCIPLE OF THE DEVICE Consider an off-axis source (such as could be produced by a wandering arc) as shown in Fig. 1. The light from this source when imaged by the lens on the plane of the slit at a’b’ misses the slit. However, a refractor plate placed in the beam could be rotated to displace the beam to image the source at ab and illuminate the slit. Furthermore, two photosensitive cells set at the edges of the area ab could detect any shift in the beam away from the slit and the signal produced could be used to operate a servo mechanism which would rotate the refract’or plate to displace the beam back to ab. Unfortunately, with many spectrographs, because of shutter, filter or step sector arrangements, it is inconvenient or impractical to mount the detectors at or even
Q
I
-,--._. I
b _a’__.---b
REFRACTOR PLATE
LENS
SLIT
BEAM
CORRECTION
- DETECTOR Fig.
AT SLIT
1.
near the slit. This results in the need for a rather more complicated arrangement regarding the positioning of the detectors and the refractor plate. This scheme is shown in Fig. 2. If the broken lines represent the beam of light from an axial source and the respective positions of the refractor plate and detectors to cause this beam to fall on the slit area ab, then it is obvious that in order to make the beam from an off-axis source (solid lines) incident upon ab the detectors must be moved as well as the refractor plate rotated. Were the detectors to remain in positions c’d’ and ey’ only partial correction would occur since they would, in effect, get in the way of the corrected beam. This is due to the fact that a refractor plate cannot bend a beam of light but only displace it, its direction remaining essentially parallel to that of the original beam. It is necessary, then, that the detectors move to positions cd and ef, a distance proportional to the displacement of the beam and thus to the angle of rotation of the refractor plate.
A device to correct the effect of arc wander
If x is t’he lateral displacement the movement the respective
1533
of the beam produced by the refractor plate, then X
where x and x + y are is ee’ = x . x -1 y distances of the detectors and the lens from the slit. necessary for the detectors
SLIT
k------t-----y+
ee’.
z.
X
x+Y
z= t sini(l-s)
BEAM CORRECTION - DETECTOR IN FRONT OF SLIT Fig. 2
For a fused quartz plate of refractive of a ray is z==tsin1,
index n and thickness t the displacement
4\ I--
cos i n cos
1
rl
For small angles z is nearly proportional to sin i, but the ratio of the cosines soon becomes appreciably less than unity, causing a somewhat more rapid increase in z as the angle increases. This means that for other than small angles of rotation the motion of the detectors should become rather complicated. However, it was found that good compensation could be achieved having the detectors merely describe a small arc. The arrangement used is shown in Fig. 3. As the refractor plate rotates, the detectors, mounted on a segment of a large gear are caused to move in the same direction as the plate. The detectors are mounted on a small track so that they can be moved with respect to the refractor plate. Also, the distance between them can be adjusted. The best position of the detector was found by trial and error, but a good estimate can be made of their approximate position and the gear ratios by using the above formulae. DESCRIPTION OF TIIE DEVICE The detectors are made from two photo sensitive cadmium sulphide resistors (Philips Electronics Industries Ltd.). Since the edge of the beam can be well defined, and in order to present a straight edge of the detector to the beam, each resistor (normally circular) is removed from its glass casing and cut in half with a jeweller’s saw. New contacts are made to the resistors with conducting paint of the type used
1534
S. S. BERMAN,D.S.RUSSELL
and L.L.T.BRADLEY
to repair printed circuit boards. A coating of “Q dope” provides protection for the exposed surfaces. The photo sensitive resistors form two sides of a bridge circuit as shown in Fig. 4. Two, 60-csquare waves, 180" out of phase, are obtained by squaring the 60-cof
TO MOTOR
DETECTOR AND REFRACTOR PLATE MECHANISM Fig. 3
the power-supply transformer secondary winding. These square waves are applied to the two cells whose outputs are combined to provide a single signal of magnitude proportional to the intensity of illumination of the detector and thus proportional to the beam position error. The phase of this signal is indicative of the direction of the error. The signal is amplified and transformer coupled to a 115V, 60-c,low inertia, phase-sensitive motor (Kollsman, Type 77602) which is geared to the refractor plate. The direction of rotation of the motor is therefore dependent upon which resistor is more conducting, and the refractor plate is caused to rotate to reposition the beam to provide equal conduction in both detectors. The entire mechanism is housed in a 10 x 18 x 10 cm case which is mounted on the optical bar of the spectrograph. In operation the detectors and refractor
A device to correct the effect of arc wander
1535
1536
S. S. BERMAN,D. S.RUSSELL and L.L.T.BRADLEY
plate are covered by a shield to reduce the influence of room lighting. The device operates quite well in a brightly lit room even without the shield as long as the detectors are equally illuminated, but variations in room illumination can understandably upset the balance of the system. The refractor plate used in the original model (to which most of the following experimental results apply) was a 1.5 cm fused silica plate of optical grade quality. This thickness was chosen in order to maintain small angles of rotation and still displace the beam through distances of 3 to 4 mm. The disadvantage of using a thick plate is that there is a substantial loss in transmission, especially below 3500 A for fused silica. A later model was built employing a fused quartz plate of Suprasil II quality (Englehard Industries Ltd.) which has an almost uniform transmission t,o beyond 2000 A. A second device was also constructed in which the detectors were fixed about the center line of the diaphram of a “three-lens system”. In this system an enlarged image of the source is focussed on a diaphram by a spherical lens. This image in turn is focussed on the spectrograph collimator by two crossed cylindrical lenses. Using this arrangement, it is not necessary for the detectors to move. The following experimental results were obtained using the more general device described earlier. Data obtained from the simpler system indicate a corresponding degree of success in increasing precision. STATIC TESTS The operation of the device was first demonstrated by a static test. A small lamp was placed in the electrode holder of the arc stand to simulate the arc and focussed on the slit. The lamp was then moved laterally with the tiller to cause the image to fall at various distances from the slit. The position of the tiller was noted for each of these distances. The device was then clamped to the optical bar. A pair of electrodes were arced and moved with the tiller to throw their image off the slit. The positions of the image after displacement by the refractor plate were determined and compared with those which would have pertained had the device not been used. Fig. 5 shows the result of two sets of such experiments. In the first the detectors were kept stationary, and in the second they were allowed to move in the manner described above. In the latter case excellent compensation can be obtained even for beam excursions as large as 3.5 mm (an extreme never attained in practice). Thus the wandering image can be kept centred on the slit at all times provided that the mechanism can react quickly enough to follow the rapid fluctuations of the arc. In fact, because of the inertia of the system total compensation is not obtained but significant improvement can be demonstrated. Use of the refractor plate does cause a change in the optical path of the beam resulting in its being focussed in front of the slit rather than in the plane of the slit. A loss in line intensity may then be expected from this source. This could be corrected by focussing the system with the refractor plate in place but was not done in our case and part of the intensity loss reported later can probably be ascribed to this factor. Also, a certain degree of astigmatism may be expected, especially at
A device
t.o correct
the effect
1537
of arc wander
large angles of rotation. However, this did not produce any visible effects on the line spectra nor on the uniformity of slit illumination when either device was employed. It should also be noted that in contrast to manual tilling the refractor plate does not return the source to the optic axis [Fig. 21 but merely ensures that the slit is illuminated and that the light enters the spectrograph. Since the light will often
+-+ / -
oT~~4~o-o-o
o-o-
/
+---+
1
I
-3
-2
+
Detectors
stationary
0
Detectors
moving
I
I
-I
0
Position
+I
of uncorrected (mm
from
Fig. 5. Beam correct,ion achieved
I
I
+2
I
+3
beam.
slit) by the device.
be entering at an angle to the optical axis the collimator is not always symmetrically or totally illuminated. This factor is also a major contribution to losses in line intensities. COMPARISOX
TESTS
If the device is effective, then one of the results would be a relative increase in the intensities of spectra due to the fact that the source is imaged more often directly on the slit. That this is indeed true was demonstrated by the following experiment. Two sets of eight replicate samples each were prepared from 0.1 per cent G Standard (Spex Industries inc.), a standard containing O-1 per cent of each of fortythree elements in a graphite base. These sets were arced at 12 A d.c. and their spectra photographed, all on the same plate. For the first set the device was in position but inoperative. The detectors were spread so as not to interfere with the beam and the refractor plate set to image an axial source on the spectrograph slit. The electrodes were manually tilled during the burns in an effort to keep the source image on the slit. For the second set the device was in operation. 10
S. S. BERMAN, D. S. RUSSELL and L. L. T. BRADLEY
1638
Spectral line intensities of eighteen elements were measured. The results are shown in Table 1. In all cases but one an increase in intensity was noted, the average increase being 22 per cent. Table 1. Effect of the device on line intensities (12 A d.c. arc) Spectral line A Tl Mn Mg Pb Sn Si Fe Ge Ni Bi Al Be MO Ti Ag Na Zn co
2767 2801 2802 2833 2839 2881 2966 3039 3050 3067 3082 3130 3170 3239 3280 3302 3387 3453
Intensity Manual
Device
19.3 22.0 5.2 6.2 29.6 23.5 Il.0 13.8 11.1 14.4 26.3 25.5 11.2 14.7 10.9 14.7 11.4 12.7 37.9 45.9 33.4 36.4 4.9 6.6 13.1 15.2 30.6 36.3 42,6 35.2 9.6 13.2 15.4 20.0 13.1 15.5 Average intensity increase
0’ /o increase 14 19 26 25 30 -3 31 35 11 21 9 35 16 19 21 38 30 18 22 per cent
The experiment was repeated with the samples arced at 7 A d.c., the lower current producing a rather erratic arc. In this case all intensities were increased with an average increase of 68 per cent. ENHANCEMENT
OF
PRECISION
An important consequence of improved tilling should be an increase in precision If the incident radiation on the slit is in spectral line intensity measurements. more closely controlled a major source of erratic results in d.c. arc measurements will have been reduced. Two sets of twenty replicate samples each were prepared from a standard containing forty-three elements (Spex Mix, Spex Industries Inc.). These were each arced at 12 A d.c. and their spectra photographed, each set on a separate plate. For the first set the device was removed from the optical bar and the image was maintained on the slit by manual tilling. For the second set the device was used to keep the source imaged on the slit. Spectral line intensities of twenty-three elements were recorded in this experiment. This represented the total number of the forty-three elements present in the sample which produced, under our conditions of excitation and illumination, spectral line images with transmittances neither too low not too high to be measured. Thus, in a way, a random sampling of elements was employed.
A device to correct the effect of arc wander
1539
The coefficient of variation for each spectral line intensity each set of measurement,s. The results are shown in Table 2.
was calculated
for
Table 2. The effect of the device on the precision of line intensities Spectral line (A) B
2497
Ge 2709 Mg Mn Pb Si Fe Cr Ni Bi Ba Al MO Sn Ca V Ti Cd Na Be Sr Zr co
2781 2798 2833 2881 2966 3017 3050 3067 3071 3082 3171 3175 3179 3185 3236 3261 3302 3343 3351 3430 3474
Coefficient of variation Manual Tilling Device 8.5 10.9 13.1 9.9 12.5 8.1 7.5 10.0 5.6 16.7 9.0 10.2 8.2 8.1 12.5 13.1 11.5 12.5 9.8 10.9 15.2 13.3 10.1 Average 10.7
7.3 6.3* 8.5* 10.1 10.4 5.0* 7.9 8.2 5.1 11.7* 7.8 9.4 11.0 5.3* 8.0* 11.0 10.2 8.9* 7.7 5.7* 15.1 13.4 7.3* 8.8
* Significant (90 per cent confidence limit) increase in precision over manual tilling.
These results indicate that the device affords a general increase in precision over manual tilling. In order to more fully evaluate this data a one-sided F-t,est [Ill was applied. In this test F is the ratio of the squares of the coefficients of variation of the two sets under comparison, with the greatest precision set being made the denominator. The test was applied at the 90 per cent level of confidence (F = 1.76) to indicate significant differences in precision. Nine elements showed a significant increase in precision for the device over manual tilling. These are marked with asterisks in Table 2. In no case was manual tilling superior to the device. The analytical spectrographer is usually more concerned with intensity ratios than with intensities alone. Table 3 shows a comparison of the coefficients of variation obtained from an array of intensity ratios. The fifteen ratios were chosen to give a set of line ratios whose relative excitation potentials varied over a large range. Ten of [l l] E. L. CROW, F. A. DAVIS and M. W. MAXFIELD, Statistics ManuaZ p. 74. (1960).
Dover, New York
S. S. BERMAN, D. S. RUSSELL and L. L. T. BRADLEY
1540 Table
3. The effect of the device
on the precision
of line ratios
Cocffioient Wavelengths (8)
E,I’%*
Bi 3067/Al 3082 Ti 3236/Zr 3438 Co 3462/Fe 2966 Ge 2709/Sn 3175 Mn 2798/Bi 3067 B 2497/Sn 3175 Pb 2614/Ge 2709 Cr 3015/Al 3082 Pb 2614/Sn 3175 Pb 2614/Bi 3067 Ca 3179/Mg 2781 Zr 3438/B 2497 Zr 3438/G@ 2709 Zr 3438/Sn 3175 Zr 3438/Ni 3461
1 .oo I.01 I.01 I.07 I.10 1.15 1.23 I.26 1.32 1.41 1.83 2.14 2.29 2.45 2.94
Manual Tilling
Average * EJE, ratio of excitation pot,entials. t Significant (90 per cent confidence limit)
18.1 12.5 12.0 14.3 9.4 11.2 14.8 9.6 12.1 8.4 9-6 13.6 24.2 21.8 18.4 14.0
increase in precision
of variation,
(o/o)
’
Device 11.6t 12.5 9.9 5.9t 6.6t 4.1t 7.97 7.7 7.3t 5.6t 8.6 10.4 9.6.f 8.2t 3.0t 7.9
over manual tilling.
these indicate a significant increase in precision when the device is used and, again, in no case was manual tilling superior to the device. It is interesting to note that even ratios of lines with widely differing excitation potentials such as the last three shown in this table which give very poor precision with manual tilling show highly significant increases in precision when the device is employed. CONCLUSIONS
A relatively simple and compact device has been designed to maintain an erratic arc imaged on a spectrograph slit. The device produces results as good as, and in many cases superior to, those obtained when manual tilling is employed. There is some over-all loss (about 20 per cent) in light intensity. The gain due to the repositioning of the beam is overshadowed by losses due to the loss of focus, partial illumination of the collimator when the beam is being corrected and absorption and reflexion losses in the refractor plate. This is not considered serious since in the great majority of work with d.c. arc excitation there is more often than not an overabundance of intensity. The device relieves the spectrographer of the chore of constantly monitoring and correcting the position of the arc image. He may now devote more of his time to other duties such as maintaining a constant electrode gap which, in itself, should also enhance the precision of his work.
Acta,
1963,
Vol.
19, pp. 1531
to 1540.
l’ergamon
Press
Ltd.
Printed inNorthern Ireland
A device to correct the effect of arc wander* S. S. BERMAN, D. S. RUSSELL and L. L. T. BRADLEY? Division of Applied Chemistry, National Research Council, Ottawa, Canada (Received
28 September
1962)
Abstract-A compact device has been developed for maint)aining a beam of light emitted from an arc discharge incident on the spectrograph slit. A photosensitive detecting system monitors the The latter repositions beam and operates a servo-mechanism which rotates a refract,or plate. t,he beam should it begin to wander off the slit. Data are presented which show a significant increase in precision when this device is used.
INTRODUCTION ONE of the drawbacks to the use of the d.c. arc and a major contributor to erratic results in emission spectroscopy is arc wandering. Many proposals such as rotating magnets [I], gas jets [Z], special electrodes [3, 41 and rotating electrodes [5] have been made in an attempt to stabilize t’he arc and thus reduce wander, but few have dealt with the actual correction of the effect of wander in cases where it does exist. The most common method for dealing with the problem is by tilling. That is, the spectrographer watches the image of the arc on the spectrograph slit and moves the electrode system laterally in order to keep the image on the slit as the arc wanders. Most of the elaborate arc-spark stands are equipped with a tiller for this purpose. ARRAK and MITTLEDORF [6] cite and describe two optical systems to correct the effects of arc wander. One, the Linsenraster [7, 81, images the source with a mosaic of lenses in such a fashion that as the arc wanders the image moves to a much smaller degree. The second, suggested by LINDER [9], utilizes a biprism which produces side by side images of the arc. As one image moves off the slit the other moves onto it. Both systems result in a considerable loss in light intensity. CAVE [lo] constructed an electronically operated electromagnet to control arc wander. The device developed by the authors is a compact system mounted on the * Presented at the Ninth Ottawa Symposium on Applied Spectroscopy, Ottawa, Canada., 18 September 1962. Issued as N.R.C. No. 7457. t Present address: Division of Pure Physics, National Research Council, Ottawa, Canada. [l] [2] [3] [4] [5] [6] [7] [8] [9] [IO]
A. W. C. M. E. A. E. E. W. G.
T. MEYERS and B. C. BRUNSTETTER, Anal. Chem. 19, 71 (1947). H. CHAMP, spez Speaker 3, 1 (1962). R. JEPPESEN, E. J. EASTMAND and H. G. LOGAN, J. Opt.Sot.Am.. 34, 313 (1944). F. HASLER and C. E. HARVEY, Ind. Eng. Chem., Anal. Ed. 15, 102 (1949). K. JAYCOX and A. E. RUEHLE, Proc. 7th Summer Conf. Spectrosc. Its Appl. 1949, p. 10. ARRAK and A. J. MITTLEDORF, Appl. Spectrosc. 13,85 (1959). PKEUSS, Heidelberger Be&r. Mineral. u. Petrog. 4, 163 (1954). PREUSS, Spectrochim. Acta 11,457 (1956). J. LINDER. Unpublished work. (See Ref. 6). C. B. CAVE, Anal. Cllem. 20, 817 (1948).
1531
1532
S. S.
BERMAN,D. S. RUSSELLand L. L. T.
BRADLEY
optical bar which uses a pair of photo conductive resistors to monitor the position of the beam. A servo mechanism rotates a quartz refractor plate to keep the beam incident upon the slit. PRINCIPLE OF THE DEVICE Consider an off-axis source (such as could be produced by a wandering arc) as shown in Fig. 1. The light from this source when imaged by the lens on the plane of the slit at a’b’ misses the slit. However, a refractor plate placed in the beam could be rotated to displace the beam to image the source at ab and illuminate the slit. Furthermore, two photosensitive cells set at the edges of the area ab could detect any shift in the beam away from the slit and the signal produced could be used to operate a servo mechanism which would rotate the refract’or plate to displace the beam back to ab. Unfortunately, with many spectrographs, because of shutter, filter or step sector arrangements, it is inconvenient or impractical to mount the detectors at or even
Q
I
-,--._. I
b _a’__.---b
REFRACTOR PLATE
LENS
SLIT
BEAM
CORRECTION
- DETECTOR Fig.
AT SLIT
1.
near the slit. This results in the need for a rather more complicated arrangement regarding the positioning of the detectors and the refractor plate. This scheme is shown in Fig. 2. If the broken lines represent the beam of light from an axial source and the respective positions of the refractor plate and detectors to cause this beam to fall on the slit area ab, then it is obvious that in order to make the beam from an off-axis source (solid lines) incident upon ab the detectors must be moved as well as the refractor plate rotated. Were the detectors to remain in positions c’d’ and ey’ only partial correction would occur since they would, in effect, get in the way of the corrected beam. This is due to the fact that a refractor plate cannot bend a beam of light but only displace it, its direction remaining essentially parallel to that of the original beam. It is necessary, then, that the detectors move to positions cd and ef, a distance proportional to the displacement of the beam and thus to the angle of rotation of the refractor plate.
A device to correct the effect of arc wander
If x is t’he lateral displacement the movement the respective
1533
of the beam produced by the refractor plate, then X
where x and x + y are is ee’ = x . x -1 y distances of the detectors and the lens from the slit. necessary for the detectors
SLIT
k------t-----y+
ee’.
z.
X
x+Y
z= t sini(l-s)
BEAM CORRECTION - DETECTOR IN FRONT OF SLIT Fig. 2
For a fused quartz plate of refractive of a ray is z==tsin1,
index n and thickness t the displacement
4\ I--
cos i n cos
1
rl
For small angles z is nearly proportional to sin i, but the ratio of the cosines soon becomes appreciably less than unity, causing a somewhat more rapid increase in z as the angle increases. This means that for other than small angles of rotation the motion of the detectors should become rather complicated. However, it was found that good compensation could be achieved having the detectors merely describe a small arc. The arrangement used is shown in Fig. 3. As the refractor plate rotates, the detectors, mounted on a segment of a large gear are caused to move in the same direction as the plate. The detectors are mounted on a small track so that they can be moved with respect to the refractor plate. Also, the distance between them can be adjusted. The best position of the detector was found by trial and error, but a good estimate can be made of their approximate position and the gear ratios by using the above formulae. DESCRIPTION OF TIIE DEVICE The detectors are made from two photo sensitive cadmium sulphide resistors (Philips Electronics Industries Ltd.). Since the edge of the beam can be well defined, and in order to present a straight edge of the detector to the beam, each resistor (normally circular) is removed from its glass casing and cut in half with a jeweller’s saw. New contacts are made to the resistors with conducting paint of the type used
1534
S. S. BERMAN,D.S.RUSSELL
and L.L.T.BRADLEY
to repair printed circuit boards. A coating of “Q dope” provides protection for the exposed surfaces. The photo sensitive resistors form two sides of a bridge circuit as shown in Fig. 4. Two, 60-csquare waves, 180" out of phase, are obtained by squaring the 60-cof
TO MOTOR
DETECTOR AND REFRACTOR PLATE MECHANISM Fig. 3
the power-supply transformer secondary winding. These square waves are applied to the two cells whose outputs are combined to provide a single signal of magnitude proportional to the intensity of illumination of the detector and thus proportional to the beam position error. The phase of this signal is indicative of the direction of the error. The signal is amplified and transformer coupled to a 115V, 60-c,low inertia, phase-sensitive motor (Kollsman, Type 77602) which is geared to the refractor plate. The direction of rotation of the motor is therefore dependent upon which resistor is more conducting, and the refractor plate is caused to rotate to reposition the beam to provide equal conduction in both detectors. The entire mechanism is housed in a 10 x 18 x 10 cm case which is mounted on the optical bar of the spectrograph. In operation the detectors and refractor
A device to correct the effect of arc wander
1535
1536
S. S. BERMAN,D. S.RUSSELL and L.L.T.BRADLEY
plate are covered by a shield to reduce the influence of room lighting. The device operates quite well in a brightly lit room even without the shield as long as the detectors are equally illuminated, but variations in room illumination can understandably upset the balance of the system. The refractor plate used in the original model (to which most of the following experimental results apply) was a 1.5 cm fused silica plate of optical grade quality. This thickness was chosen in order to maintain small angles of rotation and still displace the beam through distances of 3 to 4 mm. The disadvantage of using a thick plate is that there is a substantial loss in transmission, especially below 3500 A for fused silica. A later model was built employing a fused quartz plate of Suprasil II quality (Englehard Industries Ltd.) which has an almost uniform transmission t,o beyond 2000 A. A second device was also constructed in which the detectors were fixed about the center line of the diaphram of a “three-lens system”. In this system an enlarged image of the source is focussed on a diaphram by a spherical lens. This image in turn is focussed on the spectrograph collimator by two crossed cylindrical lenses. Using this arrangement, it is not necessary for the detectors to move. The following experimental results were obtained using the more general device described earlier. Data obtained from the simpler system indicate a corresponding degree of success in increasing precision. STATIC TESTS The operation of the device was first demonstrated by a static test. A small lamp was placed in the electrode holder of the arc stand to simulate the arc and focussed on the slit. The lamp was then moved laterally with the tiller to cause the image to fall at various distances from the slit. The position of the tiller was noted for each of these distances. The device was then clamped to the optical bar. A pair of electrodes were arced and moved with the tiller to throw their image off the slit. The positions of the image after displacement by the refractor plate were determined and compared with those which would have pertained had the device not been used. Fig. 5 shows the result of two sets of such experiments. In the first the detectors were kept stationary, and in the second they were allowed to move in the manner described above. In the latter case excellent compensation can be obtained even for beam excursions as large as 3.5 mm (an extreme never attained in practice). Thus the wandering image can be kept centred on the slit at all times provided that the mechanism can react quickly enough to follow the rapid fluctuations of the arc. In fact, because of the inertia of the system total compensation is not obtained but significant improvement can be demonstrated. Use of the refractor plate does cause a change in the optical path of the beam resulting in its being focussed in front of the slit rather than in the plane of the slit. A loss in line intensity may then be expected from this source. This could be corrected by focussing the system with the refractor plate in place but was not done in our case and part of the intensity loss reported later can probably be ascribed to this factor. Also, a certain degree of astigmatism may be expected, especially at
A device
t.o correct
the effect
1537
of arc wander
large angles of rotation. However, this did not produce any visible effects on the line spectra nor on the uniformity of slit illumination when either device was employed. It should also be noted that in contrast to manual tilling the refractor plate does not return the source to the optic axis [Fig. 21 but merely ensures that the slit is illuminated and that the light enters the spectrograph. Since the light will often
+-+ / -
oT~~4~o-o-o
o-o-
/
+---+
1
I
-3
-2
+
Detectors
stationary
0
Detectors
moving
I
I
-I
0
Position
+I
of uncorrected (mm
from
Fig. 5. Beam correct,ion achieved
I
I
+2
I
+3
beam.
slit) by the device.
be entering at an angle to the optical axis the collimator is not always symmetrically or totally illuminated. This factor is also a major contribution to losses in line intensities. COMPARISOX
TESTS
If the device is effective, then one of the results would be a relative increase in the intensities of spectra due to the fact that the source is imaged more often directly on the slit. That this is indeed true was demonstrated by the following experiment. Two sets of eight replicate samples each were prepared from 0.1 per cent G Standard (Spex Industries inc.), a standard containing O-1 per cent of each of fortythree elements in a graphite base. These sets were arced at 12 A d.c. and their spectra photographed, all on the same plate. For the first set the device was in position but inoperative. The detectors were spread so as not to interfere with the beam and the refractor plate set to image an axial source on the spectrograph slit. The electrodes were manually tilled during the burns in an effort to keep the source image on the slit. For the second set the device was in operation. 10
S. S. BERMAN, D. S. RUSSELL and L. L. T. BRADLEY
1638
Spectral line intensities of eighteen elements were measured. The results are shown in Table 1. In all cases but one an increase in intensity was noted, the average increase being 22 per cent. Table 1. Effect of the device on line intensities (12 A d.c. arc) Spectral line A Tl Mn Mg Pb Sn Si Fe Ge Ni Bi Al Be MO Ti Ag Na Zn co
2767 2801 2802 2833 2839 2881 2966 3039 3050 3067 3082 3130 3170 3239 3280 3302 3387 3453
Intensity Manual
Device
19.3 22.0 5.2 6.2 29.6 23.5 Il.0 13.8 11.1 14.4 26.3 25.5 11.2 14.7 10.9 14.7 11.4 12.7 37.9 45.9 33.4 36.4 4.9 6.6 13.1 15.2 30.6 36.3 42,6 35.2 9.6 13.2 15.4 20.0 13.1 15.5 Average intensity increase
0’ /o increase 14 19 26 25 30 -3 31 35 11 21 9 35 16 19 21 38 30 18 22 per cent
The experiment was repeated with the samples arced at 7 A d.c., the lower current producing a rather erratic arc. In this case all intensities were increased with an average increase of 68 per cent. ENHANCEMENT
OF
PRECISION
An important consequence of improved tilling should be an increase in precision If the incident radiation on the slit is in spectral line intensity measurements. more closely controlled a major source of erratic results in d.c. arc measurements will have been reduced. Two sets of twenty replicate samples each were prepared from a standard containing forty-three elements (Spex Mix, Spex Industries Inc.). These were each arced at 12 A d.c. and their spectra photographed, each set on a separate plate. For the first set the device was removed from the optical bar and the image was maintained on the slit by manual tilling. For the second set the device was used to keep the source imaged on the slit. Spectral line intensities of twenty-three elements were recorded in this experiment. This represented the total number of the forty-three elements present in the sample which produced, under our conditions of excitation and illumination, spectral line images with transmittances neither too low not too high to be measured. Thus, in a way, a random sampling of elements was employed.
A device to correct the effect of arc wander
1539
The coefficient of variation for each spectral line intensity each set of measurement,s. The results are shown in Table 2.
was calculated
for
Table 2. The effect of the device on the precision of line intensities Spectral line (A) B
2497
Ge 2709 Mg Mn Pb Si Fe Cr Ni Bi Ba Al MO Sn Ca V Ti Cd Na Be Sr Zr co
2781 2798 2833 2881 2966 3017 3050 3067 3071 3082 3171 3175 3179 3185 3236 3261 3302 3343 3351 3430 3474
Coefficient of variation Manual Tilling Device 8.5 10.9 13.1 9.9 12.5 8.1 7.5 10.0 5.6 16.7 9.0 10.2 8.2 8.1 12.5 13.1 11.5 12.5 9.8 10.9 15.2 13.3 10.1 Average 10.7
7.3 6.3* 8.5* 10.1 10.4 5.0* 7.9 8.2 5.1 11.7* 7.8 9.4 11.0 5.3* 8.0* 11.0 10.2 8.9* 7.7 5.7* 15.1 13.4 7.3* 8.8
* Significant (90 per cent confidence limit) increase in precision over manual tilling.
These results indicate that the device affords a general increase in precision over manual tilling. In order to more fully evaluate this data a one-sided F-t,est [Ill was applied. In this test F is the ratio of the squares of the coefficients of variation of the two sets under comparison, with the greatest precision set being made the denominator. The test was applied at the 90 per cent level of confidence (F = 1.76) to indicate significant differences in precision. Nine elements showed a significant increase in precision for the device over manual tilling. These are marked with asterisks in Table 2. In no case was manual tilling superior to the device. The analytical spectrographer is usually more concerned with intensity ratios than with intensities alone. Table 3 shows a comparison of the coefficients of variation obtained from an array of intensity ratios. The fifteen ratios were chosen to give a set of line ratios whose relative excitation potentials varied over a large range. Ten of [l l] E. L. CROW, F. A. DAVIS and M. W. MAXFIELD, Statistics ManuaZ p. 74. (1960).
Dover, New York
S. S. BERMAN, D. S. RUSSELL and L. L. T. BRADLEY
1540 Table
3. The effect of the device
on the precision
of line ratios
Cocffioient Wavelengths (8)
E,I’%*
Bi 3067/Al 3082 Ti 3236/Zr 3438 Co 3462/Fe 2966 Ge 2709/Sn 3175 Mn 2798/Bi 3067 B 2497/Sn 3175 Pb 2614/Ge 2709 Cr 3015/Al 3082 Pb 2614/Sn 3175 Pb 2614/Bi 3067 Ca 3179/Mg 2781 Zr 3438/B 2497 Zr 3438/G@ 2709 Zr 3438/Sn 3175 Zr 3438/Ni 3461
1 .oo I.01 I.01 I.07 I.10 1.15 1.23 I.26 1.32 1.41 1.83 2.14 2.29 2.45 2.94
Manual Tilling
Average * EJE, ratio of excitation pot,entials. t Significant (90 per cent confidence limit)
18.1 12.5 12.0 14.3 9.4 11.2 14.8 9.6 12.1 8.4 9-6 13.6 24.2 21.8 18.4 14.0
increase in precision
of variation,
(o/o)
’
Device 11.6t 12.5 9.9 5.9t 6.6t 4.1t 7.97 7.7 7.3t 5.6t 8.6 10.4 9.6.f 8.2t 3.0t 7.9
over manual tilling.
these indicate a significant increase in precision when the device is used and, again, in no case was manual tilling superior to the device. It is interesting to note that even ratios of lines with widely differing excitation potentials such as the last three shown in this table which give very poor precision with manual tilling show highly significant increases in precision when the device is employed. CONCLUSIONS
A relatively simple and compact device has been designed to maintain an erratic arc imaged on a spectrograph slit. The device produces results as good as, and in many cases superior to, those obtained when manual tilling is employed. There is some over-all loss (about 20 per cent) in light intensity. The gain due to the repositioning of the beam is overshadowed by losses due to the loss of focus, partial illumination of the collimator when the beam is being corrected and absorption and reflexion losses in the refractor plate. This is not considered serious since in the great majority of work with d.c. arc excitation there is more often than not an overabundance of intensity. The device relieves the spectrographer of the chore of constantly monitoring and correcting the position of the arc image. He may now devote more of his time to other duties such as maintaining a constant electrode gap which, in itself, should also enhance the precision of his work.