1889.] Sir W. Thomson on Electrostatic Measurement. 561 



WEEKLY EVENING MEETING, 



Friday, February 8, 1889. 



Sir Frederick Bramwell, Bart. D.C.L. F.R.S. Honorary Secretary 

 and Vice-President, in the Chair. 



Sir William ThomsoxV, D.C.L. LL,D. F.R.S. M.B.L 



Electrostatic Measurement. 



A FUNDAMENTAL requisite of a measuring instrament is that its 

 application to make a measurement shall not alter the magnitude 

 of the thing measured. When this condition is not fulfilled (as is 

 essentially the case with an electric measuring instrument not kept 

 permanently in or on the electric circuit or system to which it is 

 applied), it is the magnitude as influenced or modified by the mea- 

 suring instrument which is actually measured, and the measurement 

 is to be inter jpreted on this understanding, whatever may be the 

 circumstances. 



The nearest approach in electric measuring instruments to the 

 fulfilment of this condition, of not altering the magnitude of the 

 thing measured, is attained by the electrometer when applied to 

 measure differences of potential between different points of a wire, or 

 metallic mass of any shape, in which electricity is kej^t flowing by a 

 battery or dynamo or other electromotive apparatus. The insulation 

 of any practical electrometer is so nearly perfect that the conduction 

 of electric ty through the instrument does not sensibly diminish the 

 difference of potentials of the points touched by the electrodes, and 

 the consumption of energy is therefore j)ractically nil. In this respect, 

 therefore, the quadrant electrometer would be ideally perfect : but it 

 is only available for potentials of a few volts, and in its most sensitive 

 adjustment indicates about yi^- of a volt. The lecturer has therefore 

 designed for ordinary use in connection with electric lighting and the 

 other jDractical applications of electric energy, a series of instruments 

 which will measure by electrostatic force potentials of from 40 volts 

 to 50,000 volts. The construction of the various types of this series 

 was fully exj)lained. 



The standardisation of these instruments up to 200 or 300 volts 

 is made exceedingly easy, by aid of his ceutiampere balance and 

 continuous rheostat, with a voltaic battery of any kind, j)i*imary or 

 secondary, capable of giving a fairly steady current of yL of an ampere 

 through it and the platinoid resistance in series with it. The accuracy 

 of the electro-magnetic standardisation, within the range of the direct 

 application of this method, is quite within ^V P^^' cent. A method 

 of multiplication by aid of condensers, which was explained, gives an 

 accuracy quite within t per cent, for the measurement in volts up to 



