ON ohm's law, ,55 



This slow variation of the balance was sometimes avoided by letting the 

 batteries work until it had died away, and sometimes it was allowed for by 

 suitably arranging the order of experiment. 



As it was of considerable importance to have a battery which could bo 

 relied on for constancy for some time, six large Dauiells were charged for 

 the purpose. They were cells intended for a Thomson's battery, but were 

 fitted up for convenience with copper plates IS inches square, upon which 

 was strewed sulphate of cojjper, which again was covered with a thin layer 

 of sawdust moistened with zinc sulphate, and on the top of this was placed a 

 heavy grating of zinc. Two piles were made consisting respectively of four 

 and two of these elements, and were used in most of the experiments. The 

 internal resistance of these piles ran to about 4 and 3 ohms respectively. The 

 electromotive force was repeatedly tested during the experiments. 



At first a " Morse key " worked rapidly by the hand was tried for an alter- 

 nator; this method, though leading to no definite results, seemed to show 

 the possibility of success. Then a rotating alternator driven by hand was 

 tried ; but it was found that the results though much better were still very 

 much disturbed by the irregularities of the driving. Next a rotating alter- 

 nator was made by Mr. Garnett and fitted to a Jenkins governor ; this also 

 after repeated trials was given up, the main difficulty being that of getting up 

 sufficient speed without introducing so much resistance as to go beyond the 

 range of the governor. Some of the results got with this arrangement Avere 

 fairly good, however, and will be given below. In the arrangement adopted 

 in the final exiieriments the alternation "was managed by means of a pair of 

 electric tuning-forks. For the use of these during the Lent term I am in- 

 debted to the kindness of Dr. Michael Poster. 



Final Arrangement. 



Fig. 7, p. 56, gives a scheme of the final arrangement. AB is the 

 bridge already mentioned in the Eeport. EGG is the galvanometer circuit. 

 Between D and E and E and F are inserted two resistances of 30 ohms 

 each ; W is the fine wire, H a coil of thick German-silver wire of resistance 

 nearly equal to that of the fine wire, K a small resistance-box from which 

 twentieths could be got, the final adjustment being of course made by moving 

 the block G. D is connected with the stem of the tuning-fork PQ, whose 

 prongs are each provided with a dipper, and corresponding to the dippers 

 are two mercury-cups whose heights are adjustable. M and N are the piles 

 of four and two Dauiells. is a commutator, by means of which the smaller 

 battery can be thrown in either way, or thrown out altogether as desired. 

 One terminal of the commutator goes to the cup T, the other to F. The other 

 cup, S, is connected with one pole of the larger battery, the other pole of 

 which is connected with F through a key, L, by opening or closing which the 

 battery M may be thrown out or in at pleasure. The rest of the figure re- 

 presents an auxiliary battery, U, whose circuit goes throiigh another foi-k,V W, 

 working a break at W, and through the electromagnets of the forks VW 

 and PQ. This latter battery and fork therefore simply drive the fork PQ. 



than that corresponding to a balance ; the quick swing in the opposite direction is due to 

 tlie sudden rise of temperature causing a corresponding increase of resistance; the slow 

 return movement is due to the increase of the balancing resistance owing to the gradual 

 development of heat in the thiek wire. 



