188 Mr Wilson, On a Portable Gold-leaf Electrometer, etc. 



repeated between the successive observations. To charge to a 

 low potential, say 5 volts, the slider would be moved in until the 

 gold leaf had moved across in the negative direction over the 

 proper number of scale divisions, the earth connection would then 

 be made and the slider pulled fully out again. It is convenient 

 to make the actual measurements always with the slider com- 

 pletely out, as in this position the condenser tube is too far 

 removed from the rod to have any influence upon its potential. 

 The gold leaf reading is then entirely uninfluenced by any possible 

 failure of the condenser insulation. 



The sliding condenser is also useful as a compensator. We 

 may use it, for example, to measure charges too large to be 

 directly measured by the displacement of the gold leaf. For we 

 may always proceed by sliding in the condenser (originally fully 

 out) till the potential is reduced to zero. A reading of the scale 

 attached to the slider will give us the charge, when the instru- 

 ment has once been standardised, if the potential of the condenser 

 tube be kept always the same. The charge acquired by the gold 

 leaf system has in fact been compensated by that which has been 

 abstracted into the rod of the condenser on account of increased 

 capacity, the difference of potential remaining constant. The 

 change of capacity per mm. is found to be sensibly constant over 

 about 2 cms. of the scale, while if the condenser be moved further 

 out it falls off very rapidly. 



The electrometer stands on a substantial iron base. On one 

 of the three feet of this base is fixed an upright iron rod, to which 

 is clamped a cradle which carries the microscope. 



The displacement of the gold leaf for a change of potential 

 of one volt depends of course on the particular gold leaf used ; it 

 amounts to about ^ of a mm. per volt. With the magnifying 

 power at present attached, 6 eye-piece scale divisions = 1 volt, 

 but it would be easy to increase considerably the magnification. 

 The capacity of the gold leaf and its connections does not much 

 exceed 2 cms. 



The readings of the instrument are very steady; owing, no 

 doubt, to the double case there is apparently a complete absence 

 of disturbances due to convection currents. The gold leaf takes 

 up its position of equilibrium within a small fraction of a second, 

 so that very rapid potential changes can be followed. 



The zero remained steady when the instrument, mounted on 

 a camera tripod, was tested in the open air in a fairly high wind*. 



* I have to thank the Cambridge Scientific Instrument Co. for their help 

 in the construction of the instrument. 



