478 - REPORT— 1867. 



the instruments now supplied surpass every expectation of practical electri- 

 cians and furuisli, indeed, a ue^y engine for electrical research. 



The chief difPicultics encoimtered have heen the insulation of the Leyden 

 jar, -which has formed an essential part of all the contrivances, its main- 

 tenance at a constant potential, and the reduction to absolute measurement ; 

 in the present instrument absolutely perfect insulation is no longer required ; 

 for by a new device for converting mechanical force into statical electricity 

 (first constructed by Mr. Varley in 1859) Sir William Thomson is able at any 

 moment to replenish the jar by a few turns of a handle, and by a gauge 

 electrometer, he can insure that the same charge is constantly maintained 

 in the instrument. The difficulty of the reduction to absolute units consists 

 in the difficulty of comparing the extremely small forces produced by electro- 

 static attraction, with the force of gravitation, and in the accurate measure- 

 ment of the extremely smaU distances which separate the attracting surfaces. 

 Sir WilUam Thomson reports that these difficulties have been overcome in his 

 opinion, and that he wUl be shortly in a position to constmct and issue a 

 simple pattern of an absolute electrometer or gauge of potential which will 

 serve as a standard for general use. 



Further experiments and tests are, however, required before this eau be 

 done, as any precipitation would only injure the interests of the Committee. 

 It is right here to mention that the above experiments have been carried 

 out almost entirely at tb.c expense of Sir WiUiam Thomson. 



The repleuislier, which is founded on the principle of the electrophorus, 

 may vciy possibly supersede the old form of electrical machine entii'ely ; it 

 has some analogy with the electromagnetic machines lately invented by 

 Mr. C. W. Siemens and Professor Wheatstone, by which intense dynamic 

 effects are evolved from the smallest initial trace of magnetism, by the con- 

 version of mechanical force into electric currents, and was, indeed, sug- 

 gested by this invention to Sir "William Thomson, who reinvented the plan 

 patented by Mr. Yarley*. 



A modification of the same contrivance will allow the comparison of ex- 

 tremely minute quantities of electricity, such, indeed, as might be accumulated 

 on a pin's head ; by a series of rapid inductions a charge is accumulated on 

 the electrode of an electrometer, which may be made equal in potential to 

 that on the pin's head, but infinitely exceeding it in quantity ; the effect of 

 this charge in the electrometer can then be ohserved without difficulty, and 

 any increase or diminution in the quantity of electricity on the pin's head 

 or proof plane can be detected, and the rate of loss or increase observed. 

 The potentials to which various small bodies arc charged can also be observed 

 l)y the same method, the advantage of which consists in the fact that the 

 original charge on the body tested is imdisturbed by the tost, whereas by 

 any of the older tests the charge was altered by being touched by a proof 

 plane or by the electrode of the electrometer. 



A similar plan has already been proposed by Mr. Yarley and Sir WUliam 

 Thomson, with a water-dropping arrangernent, but the mechanical contrivance 

 is in all ways preferable. No exjjonse has been inciu'red by the Committee for 

 these instruments or experiments. 



Passing to the unit of current, the Committee regret that no experiments 

 have yet been made with the large absolute electrodynamometer constructed 

 with the funds granted by the Eoyal Society. Much difficulty has been 

 experienced in finding a sufficiently solid foundation in London, and probably 

 the instruments must be moved into the country for accurate use. 



* A similar plan was proposed by Mr. Nicbolson in 1785 : tide Phil. Trans. 



