58 



KEPORT 1876. 



the + + balances accuracy to less than a millimetre was not attained. The 

 alternator was the rotating piece made by Mr. Garnett, driven by the 

 governor, which, judging by the regularity and smallness of the oscillations 

 of its brake-wheel, went very uniformly during the whole experiment. The 

 rate of revolution was about three turns (causing as many alternations) 

 per second. The sensibilities for -f- -|- and + — were respectively about 150 

 and 45 during the experiment, so'that the large and small currents would be 

 proportional to about 97 and 52 respectively. The fine wire Avas a small 

 length of German-silver wire -^-l^ in. in diameter, whose resistance was 

 about 7"3 ohms ; and the counter-balancing resistance was 7'3 ohms, taken 

 entirely from the small resistance-box. The governor being started, the 

 batteries were set on at 4.G. 



It will be seen that with some little irreg-ularity the balance on the whole 

 went steadily to the right dui-ing some three quarters of an hour. In one 



point aU the observations agree, viz. that the -\ balance is more to the 



left by 1 to 3 centimetres than the -| — \- for the corresponding time. If AE 

 be the amoiint by which the average resistance is less for the smaller than 

 for the larger current, then taking 250 as the difference between the balances, 

 we get easily, from the formulae given abov6 (our imit of resistance being 

 the resistance of -^ millim. of the bridge-wire, i. e. ' j^Jgy ohm), 



250 



AE = — = 500 (takmg y = |). 



Now the variation in resistance of German silver being about -044 per cent. 

 per deg. Ceut., we get for 1°C. on 7-3 ohms a variation of about 430 in our 

 pi-esent units. Hence the average temperature of the thin wire Avas some- 

 thing over 1° C. less during the smaller than during the larger current. 

 Neither the magnitude of the cooling effect nor the irregularities in the 

 progression of the balance in this experiment is to be wondered at, since 

 we know that air-currents have a very powerful effect in cooling the thin 

 wire ; and here the wire was merely enclosed in a box to protect it from air- 

 gusts, but was otherwise unprotected. We ought therefore to expect very 

 little of this effect in most of the following experiments, where the alterna- 

 tions were 20 times as fast, and Avhcre the Avirc was enclosed in a narrow 

 tube protected from temperature variations. 



In the experiment next quoted, the alternations were made by means of 



