86 



Messrs. Ayrton, Mather, and Sumpner 



same distance from each of the instruments. The following 

 were the results obtained with three astatic reflecting-galva^ 

 nometers having coils of exactly the same size in each case, 

 but of very different resistance. 



Resistance of 



Galvanometer, 



in ohms. 



D. 



D/A 



D/A 



26 

 2457 

 6410 



349 

 2160 

 3360 



68-5 

 43-5 

 42 



94-4 

 95-4 

 101 



From this it would appear that, if the only thing altered in 

 a galvanometer be the gauge of wire with which it is wound, 

 the sensibility is more nearly proportional to the resistance 

 raised to the power § than to the resistance raised to the 

 power \. 



Another set of experiments was now made with four non- 

 astatic galvanometers, the coils of each of which were made in 

 the same mould, totally different, however, in shape from that 

 employed in constructing the coils of the three reflecting-gal- 

 vanometers given in the last list. When tested with the same 

 suspended system, the controlling magnet being adjusted to 

 produce the same periodic time in each case, the following 

 results were obtained : — 



Resistance of 



Galvanometer, 



in ohms. 



M, 



Fraction of 

 one degree per 

 micro-ampere. 



M/A 



M/A 



0-87 



4-30 



64-23 



399-25 



00149 

 00333 

 01120 

 0-2700 



0-01597 

 0-01604 

 0-01397 

 0-01352 



0-0157 

 0-0185 

 0-0246 

 0033 



In this case the sensibility appears to be more nearly pro- 

 portional to r* than to r v . "We have, therefore, in the large 

 table given two columns, one (No. 9) for D/r* and one 



(No. 10) for D/r y , which represent (according as r a or r* 

 may be regarded as the more accurate divisor to employ) the 

 deflexion per micro-ampere 



on a scale whose distance is 



