Mr Whiddington, The Shadow Electroscope lOQ' 



The Shadow Electroscope. By R. Whiddington, M.A., St John's 

 College. 



[Received 15 June 1920,] 



A simple form of Electrostatic Voltmeter of low capacity is 

 frequently useful in the laboratory. The instrument under descrip- 

 tion is of the gold leaf type designed primarily for class instruction 

 and while not capable Of the highest precision is yet sufficiently 

 accurate for many purposes*. 



All leaf electroscopes with which I am familiar require some 

 sort of optical system such as a microscope to view the leaf. 

 Attempts have been made to use a scale placed near the leaf for 

 measuring purposes, but when too near, disturbing electrostatic 

 effects are encountered, placed too far away parallax errors become 

 obtrusive. 



It occurred to me that the difficulties might be overcome by 

 simply throwing a shadow of the leaf on a semitransparent scale 

 some centimetres away, using a small 2-volt lamp as a source of 

 Hght. 



The first instrument made on these lines consisted of a tin 

 cigarette box with the lamp at one end, a transparent scale at the 

 other end and the gold leaf system with its insulation in the middle. 

 It was found as expected that quite a sharp shadow could be 

 obtained when the lamp filament was nearly parallel to the leaf. 



The final design of electroscope is shown in section in the 

 figure, the photographically reproduced scale, graduated in volts, 

 being shown below. It will be seen that the scale is practically 

 even from 100 to 500 above which the leaf becomes unstable. 



The quadrant shape of metal box was chosen as being most 

 likely to give an even scale and a constant capacity over its 

 working range. 



The tube (T) carries a well fitting sulphur plug fitted centrally 

 with a quartz tube down which passes the rod (R) which carries 

 the leaf within the case and a small cup at the top. 



The metal arm (A) is for clamping and tilting purposes and 

 carries an earthing terminal (E). 



Just below (A), a short side tube is arranged carrying an ebonite 

 block (B) in which a small lime coated spiral is fitted. When B 

 is pushed home the spiral finds a place behind R. Its object is, 

 when heated from a 2-volt cell, to provide a source of ions for 



* The original instrument, of which this one is the final form, was designed in 

 1919 for the Naval officers under instruction in Physics at the Cavendish 

 Laboratory. 



