- Email: [email protected]
A camera for electron diffraction
A CAMERA FOR
ELECTRON
DIFFRACTION
b y W. G. B U R G E R S and J. C. M. BASART N a t u u r k u n d i g Laboratorium der N.V. Philips' Gloeilampenfabrieken Eindhoven]Holtand
Summary A d e s c r i p t i o n is g i v e n o f a n a p p a r a t u s f o r t h e d i f f r a c t i o n o f e l e c t r o n s of medium velocity (30--50 KV), adapted to both the transition- and reflectionmethods. It enables one to investigate films prepared by evaporation within the apparatus itself and to study the influence which they undergo by heating.
In the following a short description is given of an apparatus for the diffraction of electrons of medium velocity (_+_ 30--50 KV), which has been built in this laboratory for the investigation of surfacelayers or thin films of various substances, according to the method first developed by Prof. G. P. T h o m s o n. In designing it use has been made (although mostly in a modified form) of ideas put forward by several investigators in this field 1), whereas for its actual construction we have benefitted largely by existing devices, such as fused metal-glass joints and rubber vacuum seals, which have been developed in the P h i 1 i p s' X-ray laboratory for the manufacture of (especially demountable) X-ray-tubes 2). The camera is adapted to both the transition- and reflectionmethods, it enables one to investigate films prepared by evaporation within the apparatus itself, to study the eventual reaction with definite vapours and, b y means of a heatable specimencarrier, the influence which the films undergo by heating them to temperatures of, for example, 500 ° C. By means of a roll-fihn arrangement a series of photographs can be 1) G . P . T h o m s o n a n d C . G. F r a s e r , Proc. Roy. Soc. A 12B, 641,193C; R. X ¥ i e r l , Ann. d. Phys. [5] B, 521, 1931; G. I. F i n c h and A. G. Q u a r r e l l , Proe. Roy. Soc..4 | ~ ! , 398; 1933; FI. B r a u n e und S. K n o k e, ZS. f phys. Chem. B 21, 297, 1933; H. S e e m a n n, Ergebn. d. techn. R6ntgenkunde IIT, 80, 1933. 2) We are also much obliged to Mr. H a l b e r s t a d t of Messrs. C . H . F . M i i l l e r in H a m b u r g for sending us photographs of his apparatus for electrondiffraction.
544
W . G. B U R G E R S A N D J . C. M. B A S A R T
taken without opening the apparatus, whereas, if desired, the diffraction-patterns can also be photographed from the outside by means of a transparent fluorescent screen. The construction and general appearance of the apparatus can be described with the aid of figures 1 and 2. It consists of three parts, the actual camera A, the specimen-chamber B and the diaphragmand cathode-chamber C. The vacuum-tight connections between these parts are of the type used for the demountable Metalix X-raytubes, they consist namely of the rubber-washers e, pressed by clamps d between machined metal faces. The same connection is applied to fix the hotfilament cathode u in the cathode-chamber C, and to close the chassis-holder a in the camera-partA. A. Camera : The camera is made of iron and includes a cylindrical part, terminating at one end into a rectangular box of about 3 × 8 cm cross-section, the above-mentioned chaSsis-holder a. Its outer end is provided with a glass-window b 1), upon which a thin transparent layer of a fluorescent substance has been deposited by spraying an alcoholic suspension 3) on to it. For this substance zinc silicate was chosen if the fluorescent screen served only to examine the diffraction-picture visually, while the actual photograph was afterwards taken on a film placed inside the camera. If, however, it was not advisable to open the apparatus for the insertion of a film, or if it was necessary to avoid its being inside the camera on account of its water-content (gelatine-layer !), a transparent calcium tungstate screen was used, the "blue" fluorescent light of which is strongly photographically active, and the diffraction-pattern was then photographed from the outside a) through the window b by means of an ordinary photographic camera, which method, of course, made much longer exposures necessary than for the ,,direct" photographs (see Fig. 3). As a film holder for these latter photographs either a frame containing one single-film (6 x 6
1) T h i s g l a s s - w i n d o w was sealed on A b y m e a n s of s e a l i n g - w a x , which c o n n e c t i o n can, of course, also be r e p l a c e d b y a r u b b e r w a s h e r - c o n n e c t i o n . I t is also possible to leave the w i n d o w clean, a n d to p r e p a r e the fluorescent screen on a s e p a r a t e glass disc, which m a y b e p l a y e d w i t h i n the chassis-box b y m e a n s of a f i t t i n g frame. 2) C o m p a r e G. H o i s t , J.H. de Boer, M. C. T e v e s a n d C. F. V e e n e roans, P h y s i c a 1, 301, 1934. 3) G. H o l s t , c.s., loc. cir.; see also E. R u s k a , Zs. f. P h y s . 8"1. 580, 1934.
I]URGERS AND BASAIVF, CAMERA FOR I:]I~ECTRON I)IFFI'IACTION
i
I
t
Fig. I
m Fig. 3 Mg0 (28 kV 0.1 mA 1 sec)
Fig. 4 (]aF.~ (28 kV 0.1 mA 1 sec)
Q
Fig. 5a Fig. 5 (;ohl-fc~il ¢1 direct l)hotogral)h (28 kV 0.~ mA 1 sec) b indirect ,, (24 ,, 2 mA 1 h o u r )
Fig. 51)
A CAMERA FOR ELECTRON
DIFFRACTION
545
cm) was used or a roll-film arrangement 1) c for films 6 × 9 cm, similar to the type applied in,ordinary roll-film camera's. The rolling of the film is effected b y means of a ground conical metal joint, visible in Fig. 1 ~). B. Specimen-Chamber: the essential feature of the specimenchamber, which contains the specimen-holder ~"and the evaporatingapparatus p, is its construction out of a glass-body, welded at both sides to chromium-iron flanges / for its demountable vacuum-tight connection with parts A and C. The glass allows the position of the specimen-carrier, etc., to be seen from the outside, and an easy connection of eventual accessories as well (for example a second evaporating apparatus or a gascontainer). The construction of the specimenholder will be clear in detail from the drawing and photograph. It carries at its lower end the actual specimen-carrier g, to which an up- and-down movement can be given b y rotating tile brass joint l, and an "azimuthal" movement by means of the glass joint n. When the reflection-method is used for obtaining the diffraction-pattern, the specimen-carrier consists in its simplest form of a small circular metal disc, carefully polished and the back of which is attached at an angle of 87 ° to a little rod, b y which it can be fixed in the specimen holder. Thus b y rotating the latter b y means of t h e glass-joint n, the angle of incidence of the electronicbeam with regard to the plane of the disc can be varied between 0 ° and 3 °. In order to investigate the influence of heating on the structure of layers, the simple carrier can be replaced by a minute heating device g, drawn in Fig. 2 and also visible in Fig. 1. This consists of a flat metal, cylindrical box (we used chromium-iron, 1{-cm diameter), which contains a small heating-element, consisting of a nickeliron resistance wire, coiled around a circular mica plate. The leads for the heating-current (about 1 Amp. for a temperature of 500 ° C) are the specimen-holder itself and a separate lead i leaving the glassbody at k 8). 1) C o m p a r e H . B r a u n e undS. Knoke, loc. cit. 2) B e f o r e i n s e r t i n g a r o l l - f i h n ( o r d i n a r y roll-films w e r e used) i n t o t h e " c a m e r a " , it w a s p r o v i d e d w i t h holes of ± 4 e m d i a m e t e r a t d i s t a n c e s of 4- 6 c m . I n this w a y it w a s p o s s i b l e to look a t t h e d i f f r a c t i o n - p a t t e r n o n the f l u o r e s c e n t - s c r e e n b e f o r e e v e r y s u c c e s s i v e photograph. 3) T o p r e v e n t s h o r t - c i r c u i t b e t w e e n t h e s e t w o leads, the c e n t r a l b r a s s r o d of t h e s p e c i m e n - h o l d e r is e n c l o s e d in a g l a s s - t u b e ; in the s a m e w a y t h e l e a d i, w h i c h o v e r a Physica I
35
54~
W.G.
B U R G E R S A N D ]~. C. M. B A S A R T
o~
o÷
¢'4
-
f
\
E N
U
A CAMERA FOR E L E C T R O N D I F F R A C T I O N
547
The temperature is measured b y a Pt-PtRh-thermo-couple o, the thin wires J) (75 ~t) leaving the glass-body at m. Thin layers prepared outside the apparatus may be deposited on circular discs of metal, glass, quartz, which can be clamped in a small holder z, drawn separately in Fig. 2. This clamp m a y also be used as a carrier for metal-pieces, of which the surface layers are to be investigated. For investigation of thin films b y means of the transition-method, these can, according to K i r c h n e r's 2) method, in many cases be conveniently deposited on thin celluloid sheets, supported b y brass discs provided with an aperture in the centre. Such a disc y can be mounted in the revolving carrier x, of which the "cross-section aspect" is shown in the lower left half of Fig. 2. By turning the fork w b y means of the glass-joint v in the side tube 3), the film can be investigated, under various angles of incidence (0 ° 90°). Finally, the evaporating-arrangement p consists of a tungstenfilament (eventually a spiral), surrounded b y a metal cylinder to limit the vapour-beam. It is connected to the mainbody of thespecimen-chamber b y means of a glass-joint. C. Cathode- and diaphragm-chamber. This part consists of a massiv chromium-iron body, welded to a glass cylinder (about 20 cm long), which latter serves as an insulator between the high-potential of the cathode and the earth-potential of the remainder of the apparatus. The chromium-iron body contains the diaphragm s and the pumping connections. The diaphragm is a copper tube of about ! 1 cm length, on to which at both ends caps can be screwed with holes of different size in the centre (we use 0,2 and 0,5 mm) ; the diameter of the hole in the middle of the diaphragm-tube is 1 ram, and serves for screening off the direct light of the cathode). After disconnecting parts A and B, the diaphragm can be easily removed for cleaning or for the interchanging of caps b y means of small crews in the projecting ring r. The pump-connections are again of the type used in the demountcertain part of its length has the shape of:a spiral so as not prevent the up-and-down movement of the carrier, has been insulated by a glass-tube where it passes through the brass-ring h, which is fastened to the central rod to ensure its approximate central position. 1) These too are surrounded by thin glass-tubes to prevent contact. 2) F. K i r c h n e r , Naturwiss. 18, 707, 1930. 3) This side-tube is not visible in Fig. l as it points backwards. Neither has it been indicated in the longitudinal section of the apparatus in Fig. 2 for the sake of clearness.
548
W . G. B U R G E R S
A N D J . C. M. B A S A R T
able Metalix X-ray tubes (machined chrominm-iron flanges, rubberwashers and coupling-nuts). As the only communication between parts A and B on the one side and the cathode-chamber on the other is formed by the small diaphragm-holes, two separate pump-systems are used (that for A--B evacuates via large holes in the projectingring r). Also the hot-filament cathode-system u is the same as that used in the demountable Metalix tubes: it allows a ready insertion of suitable glowing-filaments, and is provided with an adjustable diaphragmcap t, by which the width of the electronicbeam can be regulated. Four photographs, taken with the apparatus described 1), are here reproduced. Fig. 3 and 4 represent reflection-diagrams of arbitrary chosen preparations, MgO and CaF2, deposited on the flat specimencarrier resp. by holding it in the vapour of burning magnesium and by evaporating calciumfluoride from a hot filament. In Fig. 5 transmission-photographs of thin gold leaf are sbown. Fig. 5a is obtained in the usual way by direct action of the diffracted rays on a photographic film placed inside the apparatus, whereas Fig. 5b is obtained indirectly by photographing the fluorescent image produced by the rays on a transparent calcium tungstate screen from the outside through the glass-window g at the end of the camera A. 24 February 1934.
1) As h i g h - t e n s i o n a t r a n s f o r m e r ( p r i m a r y ) 50 A.C. w i t h r e c t i f y i n g v a l v e a n d s m o o t h i n g c o n d e n s e r of 1.5 [z F is used. I n series w i t h t h e t u b e is a r e s i s t a n c e of a b o u t t w o m e g o h m . P h o t o g r a p h s w e r e t a k e n a t a b o u t 28 K V b y s w i t c h i n g o n the f i l a m e n t - c u r r e n t for a b o u t I s e c o n d a f t e r t h e c o n d e n s e r h a d b e e n c h a r g e d . As a d i s c h a r g e c u r r e n t of m a x i n m l l y a b o u t 0.1 m i l l i a m p w a s used, the v o l t a g e - d r o p t h r o u g h t h e r e s i s t a n c e d i d n o t c a u s e a n a p p r e c i a b l e b r o a d e n i n g of the i n t e r f e r e n c e lines. F o r s h o r t e x p o s u r e s u s i n g m u c h s t r o n g e r c u r r e n t s , a f i l m - s h u t t e r w o u l d be r e q u i r e d , w h i c h , of c o u r s e , it w o u l d be e a s y to c o n s t r u c t in the a p p a r a t u s . O n a c c o u n t of t h e light, e n t e r i n g t h e c a m e r a via t h e c e n t r a l g l a s s - b o d y , t h e r o o m i n w h i c h t h e a p p a r a t u s is p u t u p , .is d a r k e n e d d u r i n g t h e (short) e x p o s u r e of a film (this is n o t n e c e s s a r y w h e n ro!l flims a r e used}.
ELECTRON
DIFFRACTION
b y W. G. B U R G E R S and J. C. M. BASART N a t u u r k u n d i g Laboratorium der N.V. Philips' Gloeilampenfabrieken Eindhoven]Holtand
Summary A d e s c r i p t i o n is g i v e n o f a n a p p a r a t u s f o r t h e d i f f r a c t i o n o f e l e c t r o n s of medium velocity (30--50 KV), adapted to both the transition- and reflectionmethods. It enables one to investigate films prepared by evaporation within the apparatus itself and to study the influence which they undergo by heating.
In the following a short description is given of an apparatus for the diffraction of electrons of medium velocity (_+_ 30--50 KV), which has been built in this laboratory for the investigation of surfacelayers or thin films of various substances, according to the method first developed by Prof. G. P. T h o m s o n. In designing it use has been made (although mostly in a modified form) of ideas put forward by several investigators in this field 1), whereas for its actual construction we have benefitted largely by existing devices, such as fused metal-glass joints and rubber vacuum seals, which have been developed in the P h i 1 i p s' X-ray laboratory for the manufacture of (especially demountable) X-ray-tubes 2). The camera is adapted to both the transition- and reflectionmethods, it enables one to investigate films prepared by evaporation within the apparatus itself, to study the eventual reaction with definite vapours and, b y means of a heatable specimencarrier, the influence which the films undergo by heating them to temperatures of, for example, 500 ° C. By means of a roll-fihn arrangement a series of photographs can be 1) G . P . T h o m s o n a n d C . G. F r a s e r , Proc. Roy. Soc. A 12B, 641,193C; R. X ¥ i e r l , Ann. d. Phys. [5] B, 521, 1931; G. I. F i n c h and A. G. Q u a r r e l l , Proe. Roy. Soc..4 | ~ ! , 398; 1933; FI. B r a u n e und S. K n o k e, ZS. f phys. Chem. B 21, 297, 1933; H. S e e m a n n, Ergebn. d. techn. R6ntgenkunde IIT, 80, 1933. 2) We are also much obliged to Mr. H a l b e r s t a d t of Messrs. C . H . F . M i i l l e r in H a m b u r g for sending us photographs of his apparatus for electrondiffraction.
544
W . G. B U R G E R S A N D J . C. M. B A S A R T
taken without opening the apparatus, whereas, if desired, the diffraction-patterns can also be photographed from the outside by means of a transparent fluorescent screen. The construction and general appearance of the apparatus can be described with the aid of figures 1 and 2. It consists of three parts, the actual camera A, the specimen-chamber B and the diaphragmand cathode-chamber C. The vacuum-tight connections between these parts are of the type used for the demountable Metalix X-raytubes, they consist namely of the rubber-washers e, pressed by clamps d between machined metal faces. The same connection is applied to fix the hotfilament cathode u in the cathode-chamber C, and to close the chassis-holder a in the camera-partA. A. Camera : The camera is made of iron and includes a cylindrical part, terminating at one end into a rectangular box of about 3 × 8 cm cross-section, the above-mentioned chaSsis-holder a. Its outer end is provided with a glass-window b 1), upon which a thin transparent layer of a fluorescent substance has been deposited by spraying an alcoholic suspension 3) on to it. For this substance zinc silicate was chosen if the fluorescent screen served only to examine the diffraction-picture visually, while the actual photograph was afterwards taken on a film placed inside the camera. If, however, it was not advisable to open the apparatus for the insertion of a film, or if it was necessary to avoid its being inside the camera on account of its water-content (gelatine-layer !), a transparent calcium tungstate screen was used, the "blue" fluorescent light of which is strongly photographically active, and the diffraction-pattern was then photographed from the outside a) through the window b by means of an ordinary photographic camera, which method, of course, made much longer exposures necessary than for the ,,direct" photographs (see Fig. 3). As a film holder for these latter photographs either a frame containing one single-film (6 x 6
1) T h i s g l a s s - w i n d o w was sealed on A b y m e a n s of s e a l i n g - w a x , which c o n n e c t i o n can, of course, also be r e p l a c e d b y a r u b b e r w a s h e r - c o n n e c t i o n . I t is also possible to leave the w i n d o w clean, a n d to p r e p a r e the fluorescent screen on a s e p a r a t e glass disc, which m a y b e p l a y e d w i t h i n the chassis-box b y m e a n s of a f i t t i n g frame. 2) C o m p a r e G. H o i s t , J.H. de Boer, M. C. T e v e s a n d C. F. V e e n e roans, P h y s i c a 1, 301, 1934. 3) G. H o l s t , c.s., loc. cir.; see also E. R u s k a , Zs. f. P h y s . 8"1. 580, 1934.
I]URGERS AND BASAIVF, CAMERA FOR I:]I~ECTRON I)IFFI'IACTION
i
I
t
Fig. I
m Fig. 3 Mg0 (28 kV 0.1 mA 1 sec)
Fig. 4 (]aF.~ (28 kV 0.1 mA 1 sec)
Q
Fig. 5a Fig. 5 (;ohl-fc~il ¢1 direct l)hotogral)h (28 kV 0.~ mA 1 sec) b indirect ,, (24 ,, 2 mA 1 h o u r )
Fig. 51)
A CAMERA FOR ELECTRON
DIFFRACTION
545
cm) was used or a roll-film arrangement 1) c for films 6 × 9 cm, similar to the type applied in,ordinary roll-film camera's. The rolling of the film is effected b y means of a ground conical metal joint, visible in Fig. 1 ~). B. Specimen-Chamber: the essential feature of the specimenchamber, which contains the specimen-holder ~"and the evaporatingapparatus p, is its construction out of a glass-body, welded at both sides to chromium-iron flanges / for its demountable vacuum-tight connection with parts A and C. The glass allows the position of the specimen-carrier, etc., to be seen from the outside, and an easy connection of eventual accessories as well (for example a second evaporating apparatus or a gascontainer). The construction of the specimenholder will be clear in detail from the drawing and photograph. It carries at its lower end the actual specimen-carrier g, to which an up- and-down movement can be given b y rotating tile brass joint l, and an "azimuthal" movement by means of the glass joint n. When the reflection-method is used for obtaining the diffraction-pattern, the specimen-carrier consists in its simplest form of a small circular metal disc, carefully polished and the back of which is attached at an angle of 87 ° to a little rod, b y which it can be fixed in the specimen holder. Thus b y rotating the latter b y means of t h e glass-joint n, the angle of incidence of the electronicbeam with regard to the plane of the disc can be varied between 0 ° and 3 °. In order to investigate the influence of heating on the structure of layers, the simple carrier can be replaced by a minute heating device g, drawn in Fig. 2 and also visible in Fig. 1. This consists of a flat metal, cylindrical box (we used chromium-iron, 1{-cm diameter), which contains a small heating-element, consisting of a nickeliron resistance wire, coiled around a circular mica plate. The leads for the heating-current (about 1 Amp. for a temperature of 500 ° C) are the specimen-holder itself and a separate lead i leaving the glassbody at k 8). 1) C o m p a r e H . B r a u n e undS. Knoke, loc. cit. 2) B e f o r e i n s e r t i n g a r o l l - f i h n ( o r d i n a r y roll-films w e r e used) i n t o t h e " c a m e r a " , it w a s p r o v i d e d w i t h holes of ± 4 e m d i a m e t e r a t d i s t a n c e s of 4- 6 c m . I n this w a y it w a s p o s s i b l e to look a t t h e d i f f r a c t i o n - p a t t e r n o n the f l u o r e s c e n t - s c r e e n b e f o r e e v e r y s u c c e s s i v e photograph. 3) T o p r e v e n t s h o r t - c i r c u i t b e t w e e n t h e s e t w o leads, the c e n t r a l b r a s s r o d of t h e s p e c i m e n - h o l d e r is e n c l o s e d in a g l a s s - t u b e ; in the s a m e w a y t h e l e a d i, w h i c h o v e r a Physica I
35
54~
W.G.
B U R G E R S A N D ]~. C. M. B A S A R T
o~
o÷
¢'4
-
f
\
E N
U
A CAMERA FOR E L E C T R O N D I F F R A C T I O N
547
The temperature is measured b y a Pt-PtRh-thermo-couple o, the thin wires J) (75 ~t) leaving the glass-body at m. Thin layers prepared outside the apparatus may be deposited on circular discs of metal, glass, quartz, which can be clamped in a small holder z, drawn separately in Fig. 2. This clamp m a y also be used as a carrier for metal-pieces, of which the surface layers are to be investigated. For investigation of thin films b y means of the transition-method, these can, according to K i r c h n e r's 2) method, in many cases be conveniently deposited on thin celluloid sheets, supported b y brass discs provided with an aperture in the centre. Such a disc y can be mounted in the revolving carrier x, of which the "cross-section aspect" is shown in the lower left half of Fig. 2. By turning the fork w b y means of the glass-joint v in the side tube 3), the film can be investigated, under various angles of incidence (0 ° 90°). Finally, the evaporating-arrangement p consists of a tungstenfilament (eventually a spiral), surrounded b y a metal cylinder to limit the vapour-beam. It is connected to the mainbody of thespecimen-chamber b y means of a glass-joint. C. Cathode- and diaphragm-chamber. This part consists of a massiv chromium-iron body, welded to a glass cylinder (about 20 cm long), which latter serves as an insulator between the high-potential of the cathode and the earth-potential of the remainder of the apparatus. The chromium-iron body contains the diaphragm s and the pumping connections. The diaphragm is a copper tube of about ! 1 cm length, on to which at both ends caps can be screwed with holes of different size in the centre (we use 0,2 and 0,5 mm) ; the diameter of the hole in the middle of the diaphragm-tube is 1 ram, and serves for screening off the direct light of the cathode). After disconnecting parts A and B, the diaphragm can be easily removed for cleaning or for the interchanging of caps b y means of small crews in the projecting ring r. The pump-connections are again of the type used in the demountcertain part of its length has the shape of:a spiral so as not prevent the up-and-down movement of the carrier, has been insulated by a glass-tube where it passes through the brass-ring h, which is fastened to the central rod to ensure its approximate central position. 1) These too are surrounded by thin glass-tubes to prevent contact. 2) F. K i r c h n e r , Naturwiss. 18, 707, 1930. 3) This side-tube is not visible in Fig. l as it points backwards. Neither has it been indicated in the longitudinal section of the apparatus in Fig. 2 for the sake of clearness.
548
W . G. B U R G E R S
A N D J . C. M. B A S A R T
able Metalix X-ray tubes (machined chrominm-iron flanges, rubberwashers and coupling-nuts). As the only communication between parts A and B on the one side and the cathode-chamber on the other is formed by the small diaphragm-holes, two separate pump-systems are used (that for A--B evacuates via large holes in the projectingring r). Also the hot-filament cathode-system u is the same as that used in the demountable Metalix tubes: it allows a ready insertion of suitable glowing-filaments, and is provided with an adjustable diaphragmcap t, by which the width of the electronicbeam can be regulated. Four photographs, taken with the apparatus described 1), are here reproduced. Fig. 3 and 4 represent reflection-diagrams of arbitrary chosen preparations, MgO and CaF2, deposited on the flat specimencarrier resp. by holding it in the vapour of burning magnesium and by evaporating calciumfluoride from a hot filament. In Fig. 5 transmission-photographs of thin gold leaf are sbown. Fig. 5a is obtained in the usual way by direct action of the diffracted rays on a photographic film placed inside the apparatus, whereas Fig. 5b is obtained indirectly by photographing the fluorescent image produced by the rays on a transparent calcium tungstate screen from the outside through the glass-window g at the end of the camera A. 24 February 1934.
1) As h i g h - t e n s i o n a t r a n s f o r m e r ( p r i m a r y ) 50 A.C. w i t h r e c t i f y i n g v a l v e a n d s m o o t h i n g c o n d e n s e r of 1.5 [z F is used. I n series w i t h t h e t u b e is a r e s i s t a n c e of a b o u t t w o m e g o h m . P h o t o g r a p h s w e r e t a k e n a t a b o u t 28 K V b y s w i t c h i n g o n the f i l a m e n t - c u r r e n t for a b o u t I s e c o n d a f t e r t h e c o n d e n s e r h a d b e e n c h a r g e d . As a d i s c h a r g e c u r r e n t of m a x i n m l l y a b o u t 0.1 m i l l i a m p w a s used, the v o l t a g e - d r o p t h r o u g h t h e r e s i s t a n c e d i d n o t c a u s e a n a p p r e c i a b l e b r o a d e n i n g of the i n t e r f e r e n c e lines. F o r s h o r t e x p o s u r e s u s i n g m u c h s t r o n g e r c u r r e n t s , a f i l m - s h u t t e r w o u l d be r e q u i r e d , w h i c h , of c o u r s e , it w o u l d be e a s y to c o n s t r u c t in the a p p a r a t u s . O n a c c o u n t of t h e light, e n t e r i n g t h e c a m e r a via t h e c e n t r a l g l a s s - b o d y , t h e r o o m i n w h i c h t h e a p p a r a t u s is p u t u p , .is d a r k e n e d d u r i n g t h e (short) e x p o s u r e of a film (this is n o t n e c e s s a r y w h e n ro!l flims a r e used}.