Mr Whiddington, The Shadow Electroscope 111 



The scale was graduated by applying known voltages from a 

 small direct current generator*, measuring them by a standard 

 Weston Voltmeter. 



I have found that with this instrument and the scale repro- 

 duced above, it is sufficient, when no more than approximate 

 results are required, to register the shadow of the leaf for two 

 positions only — ^zero and one other, say 200 volts. To effect this 

 it will generally be necessary to alter the sensitiveness somewhat 

 by adjusting the height of the sulphur block in T. This is no 

 doubt due to the non-uniform aluminium leaff available. 



Charging the Electroscope. 



After connecting E to earth, the leaf may be charged positively 

 by induction from a rubbed ebonite rod. If a negative charge is 

 required care should be taken not to overcharge the leaf. If an 

 appreciable leak is observed a small piece of smooth silk rubbed 

 lightly over Q will almost certainly cure it. 



Insulation troubles are nearly always traceable to hairs and 

 dust particles attracted under the comparatively high voltages used. 

 It is therefore best to conduct the experiments in a dust free room. 



The following are a few of the experiments which can be carried 

 out with this instrument. 



Experiment 1. To determine the capacity (Cg) of the electroscope 

 hy comparison with that of a sphere of radius r cm. 



Method. Charge the leaf to a voltage F^ as indicated by the 

 scale reading (with the case earthed), and then share the charge 

 on the J leaf with the insulated sphere thereby causing a drop in 

 potential to Fg. 



Then since q = Cg F^ = (Cg + ?") Fg, ? being the original charge, 



r 



The following table shows a series of measurements taken on 



* Kindly lent by the Electric Construction Company, Wolverhampton. 



t Cut with scissors from leaf approximately -0004 cms. thick. 



X It is here assumed that the capacity of the sphere is equal to its radius. This 

 is only true when the sphere is far removed from other conductors, a condition 

 which can be approximately realised in practice if a long thin stiff vertical wire 

 be inserted in the cup of the electroscope (or stalk of the condenser as the case may 

 be) and the sphere touched to the top of the wire. If this precaution be neglected 

 the results obtained will be too small. 



Further, it must be remembered that when bringing up the sphere to the 

 electroscope for charge sharing, any charge on the insulating handle will affect the 

 leaf by induction and spoil the results. This effect may be got rid of by passing 

 the handle through a flame occasionally, merely touching the ebonite is often 

 sufficient to produce a charge. 



