218 Intelligence and Miscellaneous Articles. 



difference of potential between the ends of the strip, 



JA=QE, 

 where J is the mechanical equivalent of heat. We have 



Q=¥ 2sin i' ^ 



where a is the first swing of the needle of the ballistic galvanometer, 

 Gr the galvanometer constant, T the period of a single vibration of 

 the needle, and H the horizontal component of the earth's magnetic 

 force. Gr was determined by comparison of the deflections on the 

 scale of the ballistic galvanometer with the readings of a tangent- 

 galvanometer whose constant was calculated, included in the same 



circuit. In the experiments, 2 sin _ was considered as proportional 



to S, the deflection on the scale, and the value of Gr for 2 = 1 cm. 



was found to be 769*4. A shunt was used with the galvanometer, 



r-l-S 

 so that the value of Q above given is to be multiplied by — ^— ' 



r being the resistance of the galvanometer, and S that of the shunt. 

 The arrangement of the apparatus was as follows : — 



E. The steel strip enclosed in a glass tube to protect it from 



draughts of air. 

 W. Wheatstone's bridge. 

 Gr r Thompson astatic galvanometer. 

 8 X . Shunt for the same. 

 Gr 2 . Ballistic galvanometer. 

 S a . Shunt for the same. 

 E. Quadrant-electrometer. 

 B r Battery of twelve Bunsen cells. 

 B 2 . Battery of two Leclanche cells. 

 B 3 . Do. ' do. do. 



K T . Key for battery B 2 aud galvanometer Gr r 

 K 2 . Key for passing current from B x through strip. 

 K 3 . Key in auxiliary circuit with commutator C, and a second 



coil of galvanometer Gr 2 , for bringing the needle quickly to 



rest without heating strip R. 



The two galvanometers were arranged to throw their spots of 

 light on the same scale. The key K x was first depressed, E being 

 then in the bridge circuit, and the spot of G 1 w<as brought to zero 

 by adjusting the resistance c. a was always 1000 ohms, and b 

 one ohm. On K x being raised, a sufficient extra resistance was 

 inserted in c, so that when K 2 was momentarily depressed, and K l 

 was immediately afterwards again depressed, the spot Gr x did not 

 move. The hand soon became accustomed to pressing K 2 just 

 long enough to accomplish this result. In the experiments, c had 

 to be increased from 1167 ohms by the amount of 50 ohms, and the 

 temperature of the strip accordingly rose about ten degrees. As 



