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Moving striations in neon-gas at the beginning of the discharge
MOVING STRIATIONS IN NEON-GAS AT THE BEGINNING OF THE DISCHARGE. by A. H. VAN GORCUM Laboratorium voor Technische Physiea der Technische Hoogeschool, Delft
Summary The positive column of a Neon discharge-tube was photographed a rotating camera: 1st at the moment the discharge was started, 2dly after short interruptions. Thus we can follow the form of the discharge during time.
with
§ 1. Introduction. It is well-known that the positive column in a lowpressure discharge-tube can be uniform or striated. In inert gases the striations m a y often be moving 1), if this occurs while the discharge-current is alternating, the column m a y appear in different forms. When feeding the discharge by an audio-frequency oscillator and amplifier, so that the frequency could be controlled easily, luminous balls appeared at regular distances, separated by darker parts (fig. 1). This occurred with a frequency of 500 cycles. These balls seemed
F i g . 1. p = 0 , 8 8 . i ---- 40. p : p r e s s u r e of t h e g a s , in c m of m e r c u r y i : c u r r e n t i n t h e t u b e , in m A a : s p a c e o f t i m e , d u r i n g w h i c h t h e c u r r e n t h a s b e e n i n t e r r u p t e d , in 10 - s s e c .
to be unmoving or slowly moving.. Examining the column from a rotating mirror it appeared that they were due to the fact, that 1) Moving striations have been photographed by C. S a m s o n, Zs. f. techn. Phys. 6, 281, 1925 and W. P u p p, Zs. f. techn. Phys. 15, 257, 1934.
- - 535 - -
536
A.H. VAN GORCUM
rapidly moving striations were balancing with the same frequency as the A. C. (fig. 2). Examining the column from a rotating mirror, another interesting fact appeared. When the frequency of the A. C. was very low (I or 2 cycles) we saw that every half part of a period the column started to be homogeneous. This did not occur with higher frequencies. (f.i. 50 cycles). To study this phenomenon the following experiment was carried out.
§ 2. Apparatus. A D.C. circuit contained the discharge-tube under investigation and a diode. By controlling the current in the
Fig. 2. p =0,88.
filament of the diode, the current in the discharge-tube could be adjusted. Moreover, we could be sure that at the beginning of the discharge the current in the tube did not rise above the saturation-
,,
,
-~-~Fig. 3.
.11
MOVING S T R I A T I O N S I N N E O N - G A S
5~7
current of the diode, or, which is the same, above the current in the tube when the discharge has become constant. The tube investigated had an internal diameter of 3,8 cm, a distance between the electrodes of 83 cm, and contained Neon-gas at a pressure of 0,25 cm of mercury. The cold, cylindrical electrodes were of iron and had a diameter of 2,5 cm by a height of 6 cm. The sparking potential of the discharge was about 1600 Volts; the potential, once the discharge had been established, amounted to about 350 Volts. The D. C. was obtained from a rectifier, giving an E M F of about 2000 Volts. The photographs were obtained on a rotating camera. Together with the tube investigated another tube, worked by 50 cycles A.C. was photographed, in order to obtain a time-basis. On all the pictures time increases to the right. The cathode is at the side of the time-basis. Fig. 4 shows the column at the beginning of the discharge. When the tube was in the field-angle of the camera, the discharge was
F i g . 4. p = 0,25. i = 80.
F i g . 5. p = 0,25. i =
120. a = 6,2.
started by the aid of a contact, fixed to the axis which turned the camera round. This contact acted a thyratron, inserted in the circuit of the discharge-tube. Fig. 5--10 show the column immediately before and after short interruptions of the discharge. To interrupt the current, the tube had been shunted by a circuit, containing a thyratron T and a relay R (see fig. 3). The apparatus worked as follows:
538
A. H. V A N G O R C U M
At first the interrupters b and c were closed, the thyratron was extinguished in consequence of the negative grid-potential, and a discharge took place in the tube. Then, when the tube was in the field-angle of the camera, the current in the relay R was interrupted b y the interrupter b and released the contact c about 10-2 sec. afterwards. This was due to the slowness of the mechanical system. At the same time the E M F of induction, arising in the coils of the relay R and the transformer a gave a positive potential to the grid of the thyratron. This caused a discharge in the thyratron; the tube was then short-circuited and extinguished. (The relay had a longer retardation than the thyratron). When the interrupter c opened the thyratron-circuit the discharge in the tube started again. The whole process can be represented schematically as follows: I
I I I I¢ I I r 0
I I I I I
I" 1I
I RETAROATION OF RELAY I RETAROATIONOF THYRATRON11 i
)'1 e I ,..!
2
s
4
$
6
7. ~O'SSSC.
i
I
i
I
I
I
I GRID-POTENTIALOF THYRATRONRISES
I I
~
I
I
I OISGHAROE IN I THYRATRON SETSIN
(The appropriate retardation of the thyratron was obtained b y regulating the variable resistances d and e.The c o n d e n s o r / p r e v e n t e d an arc to arise between the contact c).
§ 3. Results. Fig. 4 shows that immediately after the discharge has set in, the positive column is homogeneous. Then striations are formed; ]irst on the cathode-side o/the tube z). About 2.10 -3 sec. after the discharge has started the whole column is striated. In the first i00th of a second they move irregular, afterwards their velocity becomes constant 2). The striations do not appear at the beginning 1) T h i s is in c o n t r a d i c t i o n w i t h the o p i n i o n of R. W h i d d i n g . t o n , that the s t r i a t i o n s arise at t h e anode. ( N a t u r e 1 1 6 - - 5 0 6 - - 1 9 2 5 ) . 2) T h i s v e l o c i t y could be e s t i m a t e d , it v a r i e d w i t h the c u r r e n t from 120 to 200 m/sec. For more c o m p l e t e m e a s u r e m e n t s , we refer to W. E .1 e n b a a s, Ned. T i j d s c h r i f t v. N a t . 1, 16, 1934; B . v . M a n e n , P h y s i c a 1,968, 1934; W. P u p p , P h y s . Zs. 3 5 , 7 0 5 , 1934.
MOVING STRIATIONS IN NEON-GAS
5~9
of the discharge. The homogeneousness of the column can not be explained in the following way: When the circuit is closed and the sparking potential attained, the current in the tube increases from zero to the saturation-current of
F i g . 6. p = 0 , 2 5 .
i =
130.
a =
0,9.
Fig.
7.p=o,25.i---73.a.=l,O.
the diode (see § 1). It seems possible t h a t the current passes through a region where the column is homogeneous. Indeed, this happened when the current was very small (below 1 mA), but then the lightintensity of the column was too small to be photographed. (The limit was attained at a current of about 10 mA).
negative glow
Fig
8. p =
0.45.
i =
174
a =
0,8.
F i g . 9. p =
0,45.
i =
150.
a =
So we can say that, when the photographs show some discharge/orm (uniform column or striations) the current in the tube during that space o/time must have remained nearly constant and equal to the value of
0,8.
540
A. H. VAN GORCUM. MOVING S T R I A T I O N S IN N E O N - G A S
the current flowing in the tube after the discharge had been established. Fig. 5 shows that when the interruption of the discharge is long enough (several 1000ths of a sec.) the phenomenon is the same as described above. However, when the interruption is shorter than a thousandth of a second, striations can be seen at the startingmoment of the discharge. (Fig. 6 and 7). This proves t h a t the disturbance which gives rise to the striations, does not vanish at the same time as the current, but lasts some thousandths of a second longer.
F i g . 10. p ---- 0,45.
i ~
106. a -~ 1,4.
The time necessary for establishing the discharge (1/100 sec.), was nearly the same for all currents investigated (20--200 mA). The photographs show one unmoving striation at the negative side of the discharge. This was not the negative glow, as appears from photographs 8, 9 and 10, which were taken of another tube, 4 cm in diameter, provided with plate, iron electrodes at a distance of 61,5 cm and filled with Neon-gas at a pressure of 0,45 cm of mercury. This unmoving striation, 7 cm distant from the cathode, arises from the first moving one, whereas the following ones disappear at about 2 cm before. With smaller currents this striation is intermittent ; with increasing current, it becomes continuous. I wish to thank Prof. Dr. H. B. D o r g e 1 o for his initiative for this work, as well as for his valid advices. Also Ir. A. C. v. D o rs t e n for his aid at the experiments. Received Apr. 16, 1935.
Delft, March 1935.
Summary The positive column of a Neon discharge-tube was photographed a rotating camera: 1st at the moment the discharge was started, 2dly after short interruptions. Thus we can follow the form of the discharge during time.
with
§ 1. Introduction. It is well-known that the positive column in a lowpressure discharge-tube can be uniform or striated. In inert gases the striations m a y often be moving 1), if this occurs while the discharge-current is alternating, the column m a y appear in different forms. When feeding the discharge by an audio-frequency oscillator and amplifier, so that the frequency could be controlled easily, luminous balls appeared at regular distances, separated by darker parts (fig. 1). This occurred with a frequency of 500 cycles. These balls seemed
F i g . 1. p = 0 , 8 8 . i ---- 40. p : p r e s s u r e of t h e g a s , in c m of m e r c u r y i : c u r r e n t i n t h e t u b e , in m A a : s p a c e o f t i m e , d u r i n g w h i c h t h e c u r r e n t h a s b e e n i n t e r r u p t e d , in 10 - s s e c .
to be unmoving or slowly moving.. Examining the column from a rotating mirror it appeared that they were due to the fact, that 1) Moving striations have been photographed by C. S a m s o n, Zs. f. techn. Phys. 6, 281, 1925 and W. P u p p, Zs. f. techn. Phys. 15, 257, 1934.
- - 535 - -
536
A.H. VAN GORCUM
rapidly moving striations were balancing with the same frequency as the A. C. (fig. 2). Examining the column from a rotating mirror, another interesting fact appeared. When the frequency of the A. C. was very low (I or 2 cycles) we saw that every half part of a period the column started to be homogeneous. This did not occur with higher frequencies. (f.i. 50 cycles). To study this phenomenon the following experiment was carried out.
§ 2. Apparatus. A D.C. circuit contained the discharge-tube under investigation and a diode. By controlling the current in the
Fig. 2. p =0,88.
filament of the diode, the current in the discharge-tube could be adjusted. Moreover, we could be sure that at the beginning of the discharge the current in the tube did not rise above the saturation-
,,
,
-~-~Fig. 3.
.11
MOVING S T R I A T I O N S I N N E O N - G A S
5~7
current of the diode, or, which is the same, above the current in the tube when the discharge has become constant. The tube investigated had an internal diameter of 3,8 cm, a distance between the electrodes of 83 cm, and contained Neon-gas at a pressure of 0,25 cm of mercury. The cold, cylindrical electrodes were of iron and had a diameter of 2,5 cm by a height of 6 cm. The sparking potential of the discharge was about 1600 Volts; the potential, once the discharge had been established, amounted to about 350 Volts. The D. C. was obtained from a rectifier, giving an E M F of about 2000 Volts. The photographs were obtained on a rotating camera. Together with the tube investigated another tube, worked by 50 cycles A.C. was photographed, in order to obtain a time-basis. On all the pictures time increases to the right. The cathode is at the side of the time-basis. Fig. 4 shows the column at the beginning of the discharge. When the tube was in the field-angle of the camera, the discharge was
F i g . 4. p = 0,25. i = 80.
F i g . 5. p = 0,25. i =
120. a = 6,2.
started by the aid of a contact, fixed to the axis which turned the camera round. This contact acted a thyratron, inserted in the circuit of the discharge-tube. Fig. 5--10 show the column immediately before and after short interruptions of the discharge. To interrupt the current, the tube had been shunted by a circuit, containing a thyratron T and a relay R (see fig. 3). The apparatus worked as follows:
538
A. H. V A N G O R C U M
At first the interrupters b and c were closed, the thyratron was extinguished in consequence of the negative grid-potential, and a discharge took place in the tube. Then, when the tube was in the field-angle of the camera, the current in the relay R was interrupted b y the interrupter b and released the contact c about 10-2 sec. afterwards. This was due to the slowness of the mechanical system. At the same time the E M F of induction, arising in the coils of the relay R and the transformer a gave a positive potential to the grid of the thyratron. This caused a discharge in the thyratron; the tube was then short-circuited and extinguished. (The relay had a longer retardation than the thyratron). When the interrupter c opened the thyratron-circuit the discharge in the tube started again. The whole process can be represented schematically as follows: I
I I I I¢ I I r 0
I I I I I
I" 1I
I RETAROATION OF RELAY I RETAROATIONOF THYRATRON11 i
)'1 e I ,..!
2
s
4
$
6
7. ~O'SSSC.
i
I
i
I
I
I
I GRID-POTENTIALOF THYRATRONRISES
I I
~
I
I
I OISGHAROE IN I THYRATRON SETSIN
(The appropriate retardation of the thyratron was obtained b y regulating the variable resistances d and e.The c o n d e n s o r / p r e v e n t e d an arc to arise between the contact c).
§ 3. Results. Fig. 4 shows that immediately after the discharge has set in, the positive column is homogeneous. Then striations are formed; ]irst on the cathode-side o/the tube z). About 2.10 -3 sec. after the discharge has started the whole column is striated. In the first i00th of a second they move irregular, afterwards their velocity becomes constant 2). The striations do not appear at the beginning 1) T h i s is in c o n t r a d i c t i o n w i t h the o p i n i o n of R. W h i d d i n g . t o n , that the s t r i a t i o n s arise at t h e anode. ( N a t u r e 1 1 6 - - 5 0 6 - - 1 9 2 5 ) . 2) T h i s v e l o c i t y could be e s t i m a t e d , it v a r i e d w i t h the c u r r e n t from 120 to 200 m/sec. For more c o m p l e t e m e a s u r e m e n t s , we refer to W. E .1 e n b a a s, Ned. T i j d s c h r i f t v. N a t . 1, 16, 1934; B . v . M a n e n , P h y s i c a 1,968, 1934; W. P u p p , P h y s . Zs. 3 5 , 7 0 5 , 1934.
MOVING STRIATIONS IN NEON-GAS
5~9
of the discharge. The homogeneousness of the column can not be explained in the following way: When the circuit is closed and the sparking potential attained, the current in the tube increases from zero to the saturation-current of
F i g . 6. p = 0 , 2 5 .
i =
130.
a =
0,9.
Fig.
7.p=o,25.i---73.a.=l,O.
the diode (see § 1). It seems possible t h a t the current passes through a region where the column is homogeneous. Indeed, this happened when the current was very small (below 1 mA), but then the lightintensity of the column was too small to be photographed. (The limit was attained at a current of about 10 mA).
negative glow
Fig
8. p =
0.45.
i =
174
a =
0,8.
F i g . 9. p =
0,45.
i =
150.
a =
So we can say that, when the photographs show some discharge/orm (uniform column or striations) the current in the tube during that space o/time must have remained nearly constant and equal to the value of
0,8.
540
A. H. VAN GORCUM. MOVING S T R I A T I O N S IN N E O N - G A S
the current flowing in the tube after the discharge had been established. Fig. 5 shows that when the interruption of the discharge is long enough (several 1000ths of a sec.) the phenomenon is the same as described above. However, when the interruption is shorter than a thousandth of a second, striations can be seen at the startingmoment of the discharge. (Fig. 6 and 7). This proves t h a t the disturbance which gives rise to the striations, does not vanish at the same time as the current, but lasts some thousandths of a second longer.
F i g . 10. p ---- 0,45.
i ~
106. a -~ 1,4.
The time necessary for establishing the discharge (1/100 sec.), was nearly the same for all currents investigated (20--200 mA). The photographs show one unmoving striation at the negative side of the discharge. This was not the negative glow, as appears from photographs 8, 9 and 10, which were taken of another tube, 4 cm in diameter, provided with plate, iron electrodes at a distance of 61,5 cm and filled with Neon-gas at a pressure of 0,45 cm of mercury. This unmoving striation, 7 cm distant from the cathode, arises from the first moving one, whereas the following ones disappear at about 2 cm before. With smaller currents this striation is intermittent ; with increasing current, it becomes continuous. I wish to thank Prof. Dr. H. B. D o r g e 1 o for his initiative for this work, as well as for his valid advices. Also Ir. A. C. v. D o rs t e n for his aid at the experiments. Received Apr. 16, 1935.
Delft, March 1935.