- Email: [email protected]
Electric charge produced by rime on iced surfaces
P h y s i ca X l I I , no 8
September 1947
ELECTRIC CHARGE P R O D U C E D BY RIME ON ICED SURFACES by J. CLAY and C. KRAMER Natuurkundig Laboratorium van de Universiteit van Amsterdam
Summary The v a r i a t i o n of the c o n t a c t p o t e n t i a l of water, ice and rime to air is measured. The electrical charge on a c y l in d r i c a l vessel cooled with a massive cylinder of low t e m p e r a t u r e inside is s t u d ie d in an air-current, while rime is p r o d u c e d on the surface. The charge is positive in the b eg i n n i n g a b o u t 250 E Q / c m 2 sec. The max. p o t e n t i a l reached is + 80 millivolts. W h e n the rime appears on, the surface, the c y l i n d e r always becomes n e g a t i v e l y charged till a max. of - - 5 0 0 millivolts. A f t er a b o u t l0 min. no increase is found. During e v a p o r a t i o n or m e l t i n g of the ice no .charge is e v e r found. Very r e m a r k a b l e big positive changes are found by adiab at i c expansion, b u t only a f t e r at least t w o or three min. when the icelayer is there and the rime is growing. A f t e r a b o u t 12 min. no pos. charge can be p r o d u c e d a n y m o r e and it seems that the surface is co v er ed with a woolly layer. An e x p l a n a t i o n m a y be the c a p t u r e of i m m o b i l i z e d positive ions on the n e g a t i v e l y charged ice-needles.
§ 1. In the Meteorologische Zeitschrifl, June 1941, W. F i n de i s e n 1) published the results of an experiment on the electrical potential accompanying the formation of rime on the surface of a body. He pointed out that this phenomenon might constitute an explanation of the electric charge in thunderclouds. Shortly after this publication, E. L a n g e 2) wrote an article - - also published in M.Z. - - entitled ,,Volta-Potentialen an H20 Phasen als Quelle der Gewitterelektrizit~t", describing his experiments on phenomena closely associated with those dealt with by F i n d e i s e n. Since it appeared to us that there was some discrepancy between the two results we looked into the same questions, which may be of considerable importance for the explanation of the charge formation in thunderclouds. To this end we compared the potential N 508
ELECTRIC CHARGE PRODUCED BY RIME ON ICED SURFACES "509
of a fro st-covered surface to t h a t of the liquid surface, and found p a r t l y t h e s a m e r e s u l t as L a n g e did, b u t p a r t l y a different one. W e placed two c6pper dishes side b y side ; one of t h e m was cooled until it was frozen and covered with rime; a probe with a plate covered with ionium, was placed at some distance above the dish with water, a n d c o n n e c t e d to a sensitive electrometer. The ionium col-
1 L]2I-k~ I I
/
,
; ..---~.._ ~
,
I
I
I
/
--'.
•
i //
~:
1
I -.,t,,
I
I
II
i,
/I
#
Fig. 1. Arrangement for the measurement of the contact potential. A. is the cooled dish. B. is the dish with water. S. is the ionium probe. lector c o u l d be r e m o v e d i m m e d i a t e l y from above the w a t e r to a b o v e the o t h e r dish; as this dish cooled down we could follow the variation taking place when the w a t e r was frozen at the surface, and when it was covered with rime; while after some time we also observed the v a r i a t i o n of ~he potential when the t e m p e r a t u r e rose again. Fig. 1 shows the a r r a n g e m e n t described, and fig. 2 the variation of the p o t e n t i a l during freezing, riming, and melting. T h e r e was
510
J. CLAY AND C. KRAMER
a difference between the "~ariations we found and those found b y L a n g e : he found no difference in p o t e n t i a l as between w a t e r a n d ice, b u t did find a difference between a frozen' surface covered with rime and one w i t h o u t rime. W e o b t a i n e d a higher p o t e n t i a l for ice t h a n for water, in a g r e e m e n t with S o n c k e 3) and F a i rbrother and W o r m w e l l 4 ) and we found, in general, a lower potential for rime-covered surfaces t h a n t h a t found b y L a n g e, a s m a y be seen from the graph (fig. 2). *~00
mY ,'ZOO
p
p
8
/
-ZOO
/
____
/
J
tJu~.
-t, oo 0
10
ZO
30
t*O
So
60
?o
Fig. 2. Variation of the contact potential. p is periode of the freezing. r is periode of riming s is periode of melting. a potential above the not-cooled dish. b potential above the cooled dish. W e now cqme to F i n d e i s e n ' s e x p e r i m e n t s with cooled surfaces in a current of air. Here, F i n d e i s e n i n v a r i a b l y finds a positive electric potential, d e p e n d e n t upon the velocity of the air-current. H e used a streamlined vessel cooled b y a filling of methyl-alcohol which was cooled before in alcohol and CO 2. This vessel is placed into a t u b e circuit, in which the air is m o v e d b y means of a ventilator. The circuit we made was changed m a n y times in view of the difficulties which at first caused results to be uncertain. W e h a d a cylindrical vessel in the air-current, c o n n e c t e d with a sensitive electrometer, carefully isolated; and two opposite windows in our circulation tube, thus having the surface of the cylinder u n d e r constant observation. A most disturbing influence which c o u l d
E L E C T R I C C H A R G E P R O D U C E D BY R I M E ON I C E D S U R F A C E S
5t1
h a r d l y be obviated, was t h a t p a r t of the alcohol would overflow on the rime covering the cylinder, causing considerable changes in potential. These difficulties, however, were overcome when we cooled the vessel b y placing a solid metal cylinder, previously cooled in CO 2 and alcohol, inside the cylinder in the tube. The surface of the outer cylinder was carefully polished, so t h a t riming could be observed at once w i t h o u t a n y difficulty. The e x p e r i m e n t was repeated several times, with the same results each time: at first, and during a short time only, a small positive charge appears,
m 6
i
s
Fig. 3. Arrangement of the airtube with the cylindrical vessel. 1. airchannel, 2. cooler, 3. hygrometer, 4 airtube, 5. anemometer, 6. cylindrical vessel, 7. probe for capture of ions, 8. ventilator, 9 exhauster I0. expansion vessel, 11 en 12. electrometers, 13. mercury manometer (high expansion) 14. water manometer (small expansion). i n v a r i a b l y followed b y a higher negative charge. The negative charge during the e v a p o r a t i o n period of the rime, m e n t i o n e d b y F i n d e i s e n, we never found The average rate of charge per cm ~" of the surface was, in the rising part of the curve, 3 , 5 . 10 V -4 cm = 4 . 10-16 Coulombs cm2/sec., = 2,5. 103 E.Q., which is in accordance with F i n d e i s e n's observations. On the declining p a r t of the curve, the values of dV/dt are more divergent, and m o s t l y larger t h a n for the rising p a r t of the curve. The steepness of the declining p a r t of the curve appears to be proportional to the velocity of the aircurrent. The charge depends on the humidity. To obtain a n y t h i n g like a p r o p e r f o r m a t i o n of rime, h u m i d i t y must be > 50%. T h e t e m p e r a t u r e of the air is of little importance.
512
j . CLAY AND C. KRAMER
Fig. 3. B y connecting 6 to an electrometer we were able to measure the charging when 6 was at air-temperature; and the potential difference when 6 was covered with rime. What we usually observed, therefore, was a small positive charge only at the beginning of the riming process, and following this, a greater negative charge. This result is quite different from that obtained b y F i n d e i s e n, who found a much greater positive charge all along the line. Another difference was that we invariably found the negative charge while the rime was still growing on the cylinder. No electrical effect was found during melting. It is possible that F i n d e i s e n did not follow the variation of the rime-layer b y sight. ÷I00
;ra l," Ioo
200
rain.
-300 0
Z
~
6
Fig. 4. P o t e n t i a l of t h e c i l i n d e r d u r i n g for t w o a i r v e l o c i t i e s . S 1 1 m / s e c . S 2 1½ m / s e c .
8
riming
I0
S 1 en
12
S2 variations
W e give the following quantitative details of the charge. The sensitivity of the electrometer was 4 mV per division; the surface of the cylinder 100 cm2; the electrostatic capacity was 49 cm 4- 1,5. Different observations were made between - - 1 0 ° C and +20°C. The relative humidity was between 40 and 90 %; the velocity of the air was between 20 cm and 4 m per sec. ; usually, f m per sec. The period of positive charging varied between ½- and 3 mins. The max. potential was + 8 0 mV. The period of negative charging was followed for up to 10 mins. Max. potential, --500 mV (fig. 4) approximately. We incline to the view that this charging proceeds parallel with the difference of contact.potential between ice + air and rime + air which we also found in our first-mentioned results. In consequence
ELECTRIC CHARGE PRODUCED BY RIME ON ICED SURFACES 513
of the variation in the contact potential, first, positive, and later, negative charge m a y be taken from the air. This was, indeed, confirmed, for we found that during the growth of the rimelayer there was an extra charge of opposite sign in the air-current behind the vessel, which we detected on the body 7 (fig. 3). It was also confirmed b y the fact that the charging of the cylinder failed to occur when the air was completely cleared of all charged nuclei by means of a cotton filter. We subsequently made experiments with air compressed in cylinders and cooled down in cylinder 2 (fig. 3). On these occasions we +800
mY 600
r/
/
/
/
~,oo
200
-.zoo o
/
" ~'~'~-~ z
~
.
,o
6
rain.
8
1o
Fig. 5. P o t e n t i a l on the probe for the capture of ions d u r i n g the riming, curve T (curve p p o t e n t i a l oil the vessel).
found that, in the case of a sudden variation in the pressure, an extraordinary high charge would occur on the rimed cylinder. For this reason we investigated more closely this phenomenon of charging after adiabatic expansion of the air, and succeeded, after a few trials, in optaining exactly reproducible results. Briefly stated, these results were as follows. Charging was only observed about 2 mins. after rime had begun to form on the surface, and it was impossible to produce, it for longer than 12-14 mins. after riming had started. The charge was proportional to the variation of pregsure, and always positive. The velocity of the air-current in these experiments was generally, 0,16/sec. ; the maximum being 0,3 m/sec. To obtain the sudden increase of charge referred to, the decrease in pressure must be at least - - 1 0 cm water); we generally Physica XIII
33
514
J. CLAY AND C. KRAMER
used - - 1 4 cm water (fig. 6a). In ordel to get a subsequent charge, it is necessary to wait about 2 mins. For this reason we varied the pressure at intervals of 2 minutes. Of great importance is the ~6"00
,,oo g l l m e I
* ,qoo
,loo 2OO IO0
o, ! goo
zoa
I
T
,1
]'
!
! ! I
::ii' i
I!
I00
[ I
,.,llol o
z
I0
17.
16
18
a.
f o r 14 c m w a t e r .
12 Io
R
6
~
1o
tz
i~,
/.;
i*
t6
I
6
Z
01
o
6
b.
for
i
v
8
i
]
6
I
It.
I
lz
i
2
I !
o
I~
1o
z
I
*~. 8
6
sO V
*
i
I
#
1o t2
20.cm Hg.
I,
16
o
I I11
I z
~
c.
6
8
xo
~dn.
16
for 7 c m H g .
Fig. 6. S u d d e n p o s i t i v e v a r i a t i o n of p o t e n t i a l b y a d i a b a t i c e x p a n s i o n . T h e d a s h e d l i n e s in a. i n d i c a t e s i m u l t a n e o u s c h a r g e s c o l l e c t e d o n t h e b o d y , 7 " (fig. 3).
humidity of the air; below 50 % there is no charging. Fig. 7 shows the relation of the charge to humidity. When the expansion of the air was greater, very high charges could be prcduced. The graph in fig. 6 b and c gives the charging for a decrease in pressure of - - 20 r e s p . - 7 cm Hg. We believe these phenomena may be explained as follows. After the first layer of ice, which is very compact, ice-needles begin to form which are electrically polarized, t h e outside points of these needles having a negative potential against the inside par~s. Now
ELECTRIC CHARGE PRODUCED BY RIME ON ICED SURFACES
515
when a sudden adiabatic expansion takes place, droplets in the air will be formed on the ions, and the positive ions in the air are caught by the negatively charged points of the ice-crystals, conveying a positive charge to the cylinder. When, however, the rime has grown too much, the surface is no longer a regular field of negative needlepoints. The needles are getting irregular and form a ,,woolly" surface - - clearly noticeable - - which can no longer ,,entrap" the positive ions. This, too, m a y well be the reason why positive *t$oO
M~
/ 0/
fO00
$00
/ 0
Fig. 7. Total variation in relation to the relative humidity. ions cannot be caught until after some 3-4 mins. after the first covering of ice is formed, while this again becomes impossible after riming has proceeded too far. To these same phenomena m a y also be referred the fact that hailstones are generally electrically charged; some negatively, (from continuous charging by potential contact difference from the air in the lower parts of the cloud) and some positively (through adiabatic expansion of the air when it rises into the higher parts of the cloud). The same phenomena might also be the cause of the remarkable and important facts discovered by R o S s G u n n 5), who found that in the lower layers both droplets and hailstones are mostly negative, and in the higher layers positive; while in the zone in between the two layers both charges are found. They are, moreover, great enough to create high voltages by some mechanical separations; high enough to produce the electric fields of the lightning flashes. Received May 20th, 1947
516
ELECTRIC
CHARGE
PRODUCED
BY RIME
ON ICED SURFACES
LITTE RAT U RE 1) W. F i n d e i s e n, 0 b e r die E n t s t e h u n g d e r Gewitterelektrizit~it. Meteor. Z. 57, 6, ° 0 1 , 1940. 2) E. L a n g e, V o l t a p o t e n t i a l e a n H 2 0 - P h a s e n als Quelle d e r G e w i t t e r e l e k t r i z i t ~ i t . Meteor. Z. 57, 12, 429, 1940. 3) L. S o n c k e, E l e k t r i z i t ~ i t s e r r e g u n g z w i s e h e n E i s u n d W a s s e r . , A n n . P h y s i k . 2 8 , 5 5 1 , 1886. 4) F. F a i r b r o t h e r & F. W o r m w e l l , The e l e e t r o k i n e t i c p o t e n t i a l b e t w e e n the solid a n d lic~uid s t a t e s of a single s u b s t a n c e . J. Chem. Soe. 1 9 9 1 , 1928. 5) R o g s G u n n, The e l e c t r i c a l c h a r g e on p r e c i p i t a t i o n a t v a r i o u s a l t i t u d e s a n d i t s r e l a t i o n to t h u n d e r s t o r m s . P h y s . Rev. 71, 3, 181, 1947.
September 1947
ELECTRIC CHARGE P R O D U C E D BY RIME ON ICED SURFACES by J. CLAY and C. KRAMER Natuurkundig Laboratorium van de Universiteit van Amsterdam
Summary The v a r i a t i o n of the c o n t a c t p o t e n t i a l of water, ice and rime to air is measured. The electrical charge on a c y l in d r i c a l vessel cooled with a massive cylinder of low t e m p e r a t u r e inside is s t u d ie d in an air-current, while rime is p r o d u c e d on the surface. The charge is positive in the b eg i n n i n g a b o u t 250 E Q / c m 2 sec. The max. p o t e n t i a l reached is + 80 millivolts. W h e n the rime appears on, the surface, the c y l i n d e r always becomes n e g a t i v e l y charged till a max. of - - 5 0 0 millivolts. A f t er a b o u t l0 min. no increase is found. During e v a p o r a t i o n or m e l t i n g of the ice no .charge is e v e r found. Very r e m a r k a b l e big positive changes are found by adiab at i c expansion, b u t only a f t e r at least t w o or three min. when the icelayer is there and the rime is growing. A f t e r a b o u t 12 min. no pos. charge can be p r o d u c e d a n y m o r e and it seems that the surface is co v er ed with a woolly layer. An e x p l a n a t i o n m a y be the c a p t u r e of i m m o b i l i z e d positive ions on the n e g a t i v e l y charged ice-needles.
§ 1. In the Meteorologische Zeitschrifl, June 1941, W. F i n de i s e n 1) published the results of an experiment on the electrical potential accompanying the formation of rime on the surface of a body. He pointed out that this phenomenon might constitute an explanation of the electric charge in thunderclouds. Shortly after this publication, E. L a n g e 2) wrote an article - - also published in M.Z. - - entitled ,,Volta-Potentialen an H20 Phasen als Quelle der Gewitterelektrizit~t", describing his experiments on phenomena closely associated with those dealt with by F i n d e i s e n. Since it appeared to us that there was some discrepancy between the two results we looked into the same questions, which may be of considerable importance for the explanation of the charge formation in thunderclouds. To this end we compared the potential N 508
ELECTRIC CHARGE PRODUCED BY RIME ON ICED SURFACES "509
of a fro st-covered surface to t h a t of the liquid surface, and found p a r t l y t h e s a m e r e s u l t as L a n g e did, b u t p a r t l y a different one. W e placed two c6pper dishes side b y side ; one of t h e m was cooled until it was frozen and covered with rime; a probe with a plate covered with ionium, was placed at some distance above the dish with water, a n d c o n n e c t e d to a sensitive electrometer. The ionium col-
1 L]2I-k~ I I
/
,
; ..---~.._ ~
,
I
I
I
/
--'.
•
i //
~:
1
I -.,t,,
I
I
II
i,
/I
#
Fig. 1. Arrangement for the measurement of the contact potential. A. is the cooled dish. B. is the dish with water. S. is the ionium probe. lector c o u l d be r e m o v e d i m m e d i a t e l y from above the w a t e r to a b o v e the o t h e r dish; as this dish cooled down we could follow the variation taking place when the w a t e r was frozen at the surface, and when it was covered with rime; while after some time we also observed the v a r i a t i o n of ~he potential when the t e m p e r a t u r e rose again. Fig. 1 shows the a r r a n g e m e n t described, and fig. 2 the variation of the p o t e n t i a l during freezing, riming, and melting. T h e r e was
510
J. CLAY AND C. KRAMER
a difference between the "~ariations we found and those found b y L a n g e : he found no difference in p o t e n t i a l as between w a t e r a n d ice, b u t did find a difference between a frozen' surface covered with rime and one w i t h o u t rime. W e o b t a i n e d a higher p o t e n t i a l for ice t h a n for water, in a g r e e m e n t with S o n c k e 3) and F a i rbrother and W o r m w e l l 4 ) and we found, in general, a lower potential for rime-covered surfaces t h a n t h a t found b y L a n g e, a s m a y be seen from the graph (fig. 2). *~00
mY ,'ZOO
p
p
8
/
-ZOO
/
____
/
J
tJu~.
-t, oo 0
10
ZO
30
t*O
So
60
?o
Fig. 2. Variation of the contact potential. p is periode of the freezing. r is periode of riming s is periode of melting. a potential above the not-cooled dish. b potential above the cooled dish. W e now cqme to F i n d e i s e n ' s e x p e r i m e n t s with cooled surfaces in a current of air. Here, F i n d e i s e n i n v a r i a b l y finds a positive electric potential, d e p e n d e n t upon the velocity of the air-current. H e used a streamlined vessel cooled b y a filling of methyl-alcohol which was cooled before in alcohol and CO 2. This vessel is placed into a t u b e circuit, in which the air is m o v e d b y means of a ventilator. The circuit we made was changed m a n y times in view of the difficulties which at first caused results to be uncertain. W e h a d a cylindrical vessel in the air-current, c o n n e c t e d with a sensitive electrometer, carefully isolated; and two opposite windows in our circulation tube, thus having the surface of the cylinder u n d e r constant observation. A most disturbing influence which c o u l d
E L E C T R I C C H A R G E P R O D U C E D BY R I M E ON I C E D S U R F A C E S
5t1
h a r d l y be obviated, was t h a t p a r t of the alcohol would overflow on the rime covering the cylinder, causing considerable changes in potential. These difficulties, however, were overcome when we cooled the vessel b y placing a solid metal cylinder, previously cooled in CO 2 and alcohol, inside the cylinder in the tube. The surface of the outer cylinder was carefully polished, so t h a t riming could be observed at once w i t h o u t a n y difficulty. The e x p e r i m e n t was repeated several times, with the same results each time: at first, and during a short time only, a small positive charge appears,
m 6
i
s
Fig. 3. Arrangement of the airtube with the cylindrical vessel. 1. airchannel, 2. cooler, 3. hygrometer, 4 airtube, 5. anemometer, 6. cylindrical vessel, 7. probe for capture of ions, 8. ventilator, 9 exhauster I0. expansion vessel, 11 en 12. electrometers, 13. mercury manometer (high expansion) 14. water manometer (small expansion). i n v a r i a b l y followed b y a higher negative charge. The negative charge during the e v a p o r a t i o n period of the rime, m e n t i o n e d b y F i n d e i s e n, we never found The average rate of charge per cm ~" of the surface was, in the rising part of the curve, 3 , 5 . 10 V -4 cm = 4 . 10-16 Coulombs cm2/sec., = 2,5. 103 E.Q., which is in accordance with F i n d e i s e n's observations. On the declining p a r t of the curve, the values of dV/dt are more divergent, and m o s t l y larger t h a n for the rising p a r t of the curve. The steepness of the declining p a r t of the curve appears to be proportional to the velocity of the aircurrent. The charge depends on the humidity. To obtain a n y t h i n g like a p r o p e r f o r m a t i o n of rime, h u m i d i t y must be > 50%. T h e t e m p e r a t u r e of the air is of little importance.
512
j . CLAY AND C. KRAMER
Fig. 3. B y connecting 6 to an electrometer we were able to measure the charging when 6 was at air-temperature; and the potential difference when 6 was covered with rime. What we usually observed, therefore, was a small positive charge only at the beginning of the riming process, and following this, a greater negative charge. This result is quite different from that obtained b y F i n d e i s e n, who found a much greater positive charge all along the line. Another difference was that we invariably found the negative charge while the rime was still growing on the cylinder. No electrical effect was found during melting. It is possible that F i n d e i s e n did not follow the variation of the rime-layer b y sight. ÷I00
;ra l," Ioo
200
rain.
-300 0
Z
~
6
Fig. 4. P o t e n t i a l of t h e c i l i n d e r d u r i n g for t w o a i r v e l o c i t i e s . S 1 1 m / s e c . S 2 1½ m / s e c .
8
riming
I0
S 1 en
12
S2 variations
W e give the following quantitative details of the charge. The sensitivity of the electrometer was 4 mV per division; the surface of the cylinder 100 cm2; the electrostatic capacity was 49 cm 4- 1,5. Different observations were made between - - 1 0 ° C and +20°C. The relative humidity was between 40 and 90 %; the velocity of the air was between 20 cm and 4 m per sec. ; usually, f m per sec. The period of positive charging varied between ½- and 3 mins. The max. potential was + 8 0 mV. The period of negative charging was followed for up to 10 mins. Max. potential, --500 mV (fig. 4) approximately. We incline to the view that this charging proceeds parallel with the difference of contact.potential between ice + air and rime + air which we also found in our first-mentioned results. In consequence
ELECTRIC CHARGE PRODUCED BY RIME ON ICED SURFACES 513
of the variation in the contact potential, first, positive, and later, negative charge m a y be taken from the air. This was, indeed, confirmed, for we found that during the growth of the rimelayer there was an extra charge of opposite sign in the air-current behind the vessel, which we detected on the body 7 (fig. 3). It was also confirmed b y the fact that the charging of the cylinder failed to occur when the air was completely cleared of all charged nuclei by means of a cotton filter. We subsequently made experiments with air compressed in cylinders and cooled down in cylinder 2 (fig. 3). On these occasions we +800
mY 600
r/
/
/
/
~,oo
200
-.zoo o
/
" ~'~'~-~ z
~
.
,o
6
rain.
8
1o
Fig. 5. P o t e n t i a l on the probe for the capture of ions d u r i n g the riming, curve T (curve p p o t e n t i a l oil the vessel).
found that, in the case of a sudden variation in the pressure, an extraordinary high charge would occur on the rimed cylinder. For this reason we investigated more closely this phenomenon of charging after adiabatic expansion of the air, and succeeded, after a few trials, in optaining exactly reproducible results. Briefly stated, these results were as follows. Charging was only observed about 2 mins. after rime had begun to form on the surface, and it was impossible to produce, it for longer than 12-14 mins. after riming had started. The charge was proportional to the variation of pregsure, and always positive. The velocity of the air-current in these experiments was generally, 0,16/sec. ; the maximum being 0,3 m/sec. To obtain the sudden increase of charge referred to, the decrease in pressure must be at least - - 1 0 cm water); we generally Physica XIII
33
514
J. CLAY AND C. KRAMER
used - - 1 4 cm water (fig. 6a). In ordel to get a subsequent charge, it is necessary to wait about 2 mins. For this reason we varied the pressure at intervals of 2 minutes. Of great importance is the ~6"00
,,oo g l l m e I
* ,qoo
,loo 2OO IO0
o, ! goo
zoa
I
T
,1
]'
!
! ! I
::ii' i
I!
I00
[ I
,.,llol o
z
I0
17.
16
18
a.
f o r 14 c m w a t e r .
12 Io
R
6
~
1o
tz
i~,
/.;
i*
t6
I
6
Z
01
o
6
b.
for
i
v
8
i
]
6
I
It.
I
lz
i
2
I !
o
I~
1o
z
I
*~. 8
6
sO V
*
i
I
#
1o t2
20.cm Hg.
I,
16
o
I I11
I z
~
c.
6
8
xo
~dn.
16
for 7 c m H g .
Fig. 6. S u d d e n p o s i t i v e v a r i a t i o n of p o t e n t i a l b y a d i a b a t i c e x p a n s i o n . T h e d a s h e d l i n e s in a. i n d i c a t e s i m u l t a n e o u s c h a r g e s c o l l e c t e d o n t h e b o d y , 7 " (fig. 3).
humidity of the air; below 50 % there is no charging. Fig. 7 shows the relation of the charge to humidity. When the expansion of the air was greater, very high charges could be prcduced. The graph in fig. 6 b and c gives the charging for a decrease in pressure of - - 20 r e s p . - 7 cm Hg. We believe these phenomena may be explained as follows. After the first layer of ice, which is very compact, ice-needles begin to form which are electrically polarized, t h e outside points of these needles having a negative potential against the inside par~s. Now
ELECTRIC CHARGE PRODUCED BY RIME ON ICED SURFACES
515
when a sudden adiabatic expansion takes place, droplets in the air will be formed on the ions, and the positive ions in the air are caught by the negatively charged points of the ice-crystals, conveying a positive charge to the cylinder. When, however, the rime has grown too much, the surface is no longer a regular field of negative needlepoints. The needles are getting irregular and form a ,,woolly" surface - - clearly noticeable - - which can no longer ,,entrap" the positive ions. This, too, m a y well be the reason why positive *t$oO
M~
/ 0/
fO00
$00
/ 0
Fig. 7. Total variation in relation to the relative humidity. ions cannot be caught until after some 3-4 mins. after the first covering of ice is formed, while this again becomes impossible after riming has proceeded too far. To these same phenomena m a y also be referred the fact that hailstones are generally electrically charged; some negatively, (from continuous charging by potential contact difference from the air in the lower parts of the cloud) and some positively (through adiabatic expansion of the air when it rises into the higher parts of the cloud). The same phenomena might also be the cause of the remarkable and important facts discovered by R o S s G u n n 5), who found that in the lower layers both droplets and hailstones are mostly negative, and in the higher layers positive; while in the zone in between the two layers both charges are found. They are, moreover, great enough to create high voltages by some mechanical separations; high enough to produce the electric fields of the lightning flashes. Received May 20th, 1947
516
ELECTRIC
CHARGE
PRODUCED
BY RIME
ON ICED SURFACES
LITTE RAT U RE 1) W. F i n d e i s e n, 0 b e r die E n t s t e h u n g d e r Gewitterelektrizit~it. Meteor. Z. 57, 6, ° 0 1 , 1940. 2) E. L a n g e, V o l t a p o t e n t i a l e a n H 2 0 - P h a s e n als Quelle d e r G e w i t t e r e l e k t r i z i t ~ i t . Meteor. Z. 57, 12, 429, 1940. 3) L. S o n c k e, E l e k t r i z i t ~ i t s e r r e g u n g z w i s e h e n E i s u n d W a s s e r . , A n n . P h y s i k . 2 8 , 5 5 1 , 1886. 4) F. F a i r b r o t h e r & F. W o r m w e l l , The e l e e t r o k i n e t i c p o t e n t i a l b e t w e e n the solid a n d lic~uid s t a t e s of a single s u b s t a n c e . J. Chem. Soe. 1 9 9 1 , 1928. 5) R o g s G u n n, The e l e c t r i c a l c h a r g e on p r e c i p i t a t i o n a t v a r i o u s a l t i t u d e s a n d i t s r e l a t i o n to t h u n d e r s t o r m s . P h y s . Rev. 71, 3, 181, 1947.