ON STANDARDS OF ELECTRICAL RESISTANCE. 479 



A portable electroclj-namometer has been constructed -which will be suit- 

 able for distribution as a standard instrument. It can be compared 'with the 

 large absolute instrument, and can also be compared directly with the most 

 sensitive astatic galvanometers yet made, as has been already proved by expe- 

 riment. These instruments cannot be distributed until further experiments 

 on their constancy have been made. 



Sir William Thomson, at his own expense, has also constructed an electro- 

 dynamometer for absolute measure. His results will check those obtained in 

 London, and the portable standard will also be tested by being sent backwards 

 and forwards between Glasgow and London, to be compared alternately with 

 the absolute instruments. 



The determination of " r," the ratio between the electrostatic and electro- 

 magnetic units, is also an object pursued by the Committee. Sir William 

 Thomson has made preliminary experiments, and has obtained numbers for 

 this constant by the aid of the absolute elcctrodynamometer, and the absolute 

 electrometer already named. The number he has obtained differs so consider- 

 ably from that hitherto received that he prefei's to extend his experiments 

 before publication. The same remark applies to the measurement of the 

 electromotive force of a Daniell's cell, made by the absolute electrometer. 



It is hoped that the present lleport contains satisfactory evidence that valu- 

 able work is being done by the Committee, and that the sums of money 

 KberaUy granted by the Association have been expended on proper objects. 



It will be seen that these grants have stimulated further expenditure on 

 the part of more than one member ; and thanks are also due to the Electric 

 and International Telegraph Company, for the liberality with which they 

 have lent large batteries, thereby sa'S'ing much expense. The Committee are 

 willing to be reappointed, and require no grant of money for the ensuing 

 year. 



APPENDIX. 

 I. On a " Resistance-Measurer." Bij C. W. Siejtens, F.R.S. 



For the measurement of small resistances the method formerly employed 

 was that of the tangent galvanometer, which method is still valuable in the 

 determination of resistances which are inseparable from a difference of electric 

 potential, such, for instance, as a galvanic element. 



In measuring wire-resistance, more accurate and convenient methods have 

 been devised, amongst which that of the common differential galvanometer 

 and that known as Wheatstone's balance hold the most prominent places. 



But both these systems have disadvantages which render them insufficient 

 in a great many cases. For instance, in the first method a well-adjusted vari- 

 able-resistance-coil is necessary, which, if the method is intended to be appli- 

 cable between wide limits, wUl have impracticable large dimensions. The 

 bridge method, though very beautiful, requu-es three adjusted coils, and fre- 

 quently gives rise to calculations, which renders it unavailable for unskilled 

 operators. The sine method, which is the most siutable for measuring great 

 resistances, requires even a superior amount of skill and mathematical know- 

 ledge on the part of the operator. 



Many years' experience of these methods made me feel the want of an in- 

 strument which would, by its simplicity of construction and case of manipula- 

 tion, be capable of employment by an unskilled operator with a degree of 

 exactness equal to that of the bridge method. , 



