January 19, 1905] 



NA TURE 



275 



and if the operation has been successfully carried out 

 the result will be as indicated in Fig. i B. 



In this sort of work it is often necessary to make 

 sulphur stoppers, &-c., of various shapes. To do this 

 it is only necessary to make paper models of the re- 

 quired shape, into which the sulphur is cast. The 

 paper generally sticks to the sulphur, but may be 

 taken off with a clean knife without impairing the 

 insulation. It is advisable to do this, and also any 

 cutting away of the sulphur that may be necessary, 

 immediately after it has set, since it becomes very hard 

 and brittle soon afterwards. 



For ordinary work with radio-active substances it 

 is not necessary to employ the most sensitive type of 

 electroscope, and for such work the design shown in 

 Fig. 2 is very convenient. It consists of a brass 

 cylinder of about the proportions shown and lo cm. 

 high. The top is closed by a flat plate with a narrow 

 tubular opening a, into which a sulphur stopper b, 

 cast as above, fits fairly tightly. The sulphur is best 

 cast round the wire destined to carry the gold leaf. 

 For examining the properties of various radiations the 

 bottom may be made in the form of a ring, as shown. 

 This is fixed by the slot and pin indicated or some 



F,s 



Fiq 2 



similar arrangement, and the circular hole in the base 

 can be covered with sheets of foil, &c., if it is desired 

 to examine the penetrating power of the rays under 

 investigation. In all these instruments a hole has to 

 be cut in the metal both in front and behind the gold 

 leaf to illuminate it and to read its position. The 

 holes are conveniently of about the relative size shown ; 

 they mav be covered up with glass, mica, or trans- 

 parent celluloid, whichever is most convenient. A 

 suitable illumination is obtained by placing a sheet of 

 white paper in front of a paraffin lamp about twelve 

 inches behind the electroscope. The movement of the 

 leaves is most conveniently read by means of a micro- 

 scope of about 6 cm. focal length furnished with a 

 micrometer eye-piece. It is advisable to have a micro- 

 scope with as short a focal length as possible to in- 

 crease the magnification, and therefore the sensitive- 

 ness. 



The final appearance of the electroscope will de- 

 pend very much on the appliances at the disposal of 

 the experimenter. An instrument of this character 

 could quite well be made out of a cigarette tin, but it 

 would probably be more satisfactorv to have the metal 

 parts made by a competent mechanic. 



If cells are not available the above instrument is 



NO. 183 8. VOL. 71] 



readily charged by allowing a rubbed sealing wax or 

 ebonite rod to spark to the outside wire. In measur- 

 ing leaks the gold leaf should always be charged to 

 about the same extent, as the sensitiveness depends a 

 good deal on the amount of the deflection. The 

 instrument will not keep its charge indefinitely, but 

 will show a small leak even if no radio-active sub- 

 stances are present; this is nearly all due to the so- 

 called spontaneous ionisation of the air. There is 

 practically no leakage across the sulphur if the instru- 

 ment is properly made. 



For some purposes a more convenient arrangement 

 is that indicated in Fig. 3, where the figure is drawn 

 so as to exhibit the electroscope in its most sensitive 

 form, i.e. with the minimum capacity. A piece about 

 4 cm. deep is cut off a wide brass cylinder, and the 

 side tubes fitted on as shown. The gold leaf is carried 

 by the wire fe, and is insulated by the sulphur bead a, 

 formed in the manner already described. Thus the 

 insulation leak can only take place to the support c, 

 and can be entirely prevented by keeping c at the same 



potential as b by means of cells. The insulation of 

 the wire c from the tube which supports it need not 

 be of a very high order; it is sufficient to fix it in with 

 a rubber stopper in the manner shown. So far we 

 have all our charged system enclosed, so that there 

 arises the difficultv of charging it. This is done by 

 means of the wire d, which can be rotated about an 

 axis through the centre of the ebonite stopper e. It 

 is advisable to remove the wire d from the gold-leaf 

 system when once this has been charged. By means 

 of the sealing-wax handle / this may be accomplished 

 without discharging the electroscope. The instrument 

 is so far open. It is conveniently closed by two squares 

 of window glass cemented on to the brass cylinder 

 with sealing wax. The whole of the outside is then 

 covered with thin lead sheet or tin foil to obviate effects 

 due to the glass getting charged. Suitable windows 

 must be cut in this to allow the position of the gold 

 leaf to be read. 



The above arrangement is as sensitive as this type 

 of instrument can conveniently be made, since its 

 capacity is only that of a short piece of wire and the 



