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



On a Portable Gold-leaf Electrometer for Low or High Poten- 

 tials, and its application to Measurements in Atmospheric Electricity. 

 By C. T. R. Wilson, M.A., F.R.S., Sidney Sussex College. 



[Read 30 October 1905.] 



The instrument has an outer earth-connected case and an 

 inner insulated case supported on the neck of a small quartz 

 Leyden jar, with the inner coat of which it is connected, the outer 

 coat being connected to the outer case of the instrument. For 

 ordinary use the inner case is charged to a positive potential of 

 about 50 volts. The insulating power of the Leyden jar is very 

 high, so that in spite of its comparatively small capacity (less than 

 100 cms.) the potential does not fall more than a small fraction of 

 a volt in 24 hours. 



When the gold leaf is earthed its potential differs from that of 

 the inner case by 50 volts, and is therefore in a deflected position. 

 If we raise its potential the deflection will diminish (the charge of 

 the inner case being positive), if we give it a negative potential 

 the deflection will increase. If we adjust the observing microscope 

 so that the gold leaf is near the centre of its micrometer scale 

 when its potential is zero, the instrument is in condition to 

 measure potentials in the neighbourhood of zero up to about 

 5 volts positive or negative. For greater potentials the leaf will 

 be outside the field of view of the microscope ; over the range 

 available the scale is sensibly uniform. If the gold leaf be raised 

 to a gradually increasing positive potential it will continually fall 

 and will pass beyond the scale of the microscope when a potential 

 of a few volts has been attained. When the potential has risen 

 to 50 volts, that of the surrounding case, the deflection is a 

 minimum, and beyond this point it will again increase. The gold 

 leaf will come into sight again in the microscope when the poten- 

 tial reaches within a few volts of 100, and at 100 volts will be the 

 same as for zero potential, the difference of potential between leaf 

 and case being now the same as at first, the sign being, however, 

 reversed. The instrument is then available over a second range 

 of potentials, say from + 95 to + 105 volts. The displacement 

 of the leaf for an increase of potential of one volt will be the 

 same as in the neighbourhood of zero, the direction of the move- 

 ment, however, being opposite. 



