ON THE B.A. UNITS OF ELECTRICAL RESISTANCE. 



17 



tubes for mercury units. The flat coil we used and found very couvenient, 

 both from its shape and on account of its small variatiou-coefticieut, 

 which was only 3i per deg. Gent, in the above-mentioned units. 

 The case contains altogether nine unit coils, viz. : — 



2 Pt Ir Nos. 2 and 3. 



2 Au Ag „ 57 and 53. 



2 Pt „ 35 and 3(3. 



3 Pt Ag „ 6, 43, and 29. 



Of the first six, all except 57, which we have not measured, are mentioned at 

 p. 146 of the Reports, but none of them have proper labels. All, however, 

 were marked in some way or other so as to be identifiable. Of the last three 

 all have labels, which are complete in 6 and 43. Nos. and 21) do not 

 appear in the Reports. The temperature on 43, which does appear, agrees 

 with that given on p. 146. We used 29 as a companion middle coil to 43, 

 because its variation-coefficient was small and nearly equal to that of 43 ; 

 but otherwise we have not bestowed much care on it. 



Coils measured. — The coils which we have measured are, therefore, 2, 3, 

 58, 35, 36, 29, 43. These we call for convenience A, B, C, D, E, F, G. 

 The normal coil is the flat coil at 10° Centigrade. This temperature is 

 chosen because it was the lower limit of the temperature of the tap-water, 

 which varied on diflerent days from 10° to 12°, though it was very constant 

 during a good part of any one day. As far as our experience went, the use 

 of a stream of tap-water was the best as well as most convenient way of 

 reducing the coils to a known temperature *. 



Results of comparison : the first statement. — The following Table exhibits our 

 results in "the way which lies nearest the method by which they were ob- 

 tained : — 



R stands' for resistance of flat coil at 10° C. 



X „ „ of the respective coils A, B, &c. at 10° C. 



\ „ variation- coefficients. 



Second statement. — The above is the most convenient form of representing 

 our results ; but for the sake of comparison we give also the following (Y 

 now stands for the resistance of the coils A, B, C, &c., at the temperatures, 

 or at some one of them, given at p. 483, B.A. Report, 1867 1) : — 



* One of us, in endeavouring to find the conductivity of paraffin, has since found that 

 the teuiperature of a wire imbedded in a much greater thickness of paraffin thixn thei-e is 

 in the B.A. coils, reaches the temperature of the tap-water in considerably less than an 

 hour, the paraffin-jacket having been at a temperature of about 30° throughout to start 

 with. . t Eeprint, p. 14(3. 



]876. c 



