1896.] Scientific Instruments used by the late Dr. Joule. 



349 



11 Many careful observations of the horizontal intensity by an im- 

 proved method on Gauss and Weber's system were made alternately 

 with observations of the deflections of a tangent galvanometer and 

 the weighings of the current meter when the same currents traversed 

 each in succession. Then calling the horizontal intensity H, the 

 angle of deflection 0, and the weighing w, there was obtained a con- 

 stant 



fc = H ^ g = 0-17876. 



V W 



" Hence with these instruments 



0*17676 \/ w 



II = 



:an 



" The experiments for the determination of the horizontal intensity 

 could be effected in a few minutes." (' Collected Papers,' vol. 1, 

 p. 545.) 



Of course, as soon as H is known the current is easily measured 

 by the tangent galvanometer, for we have the usual expression 



Current = ?^ tan 6, 



where r is the radius of the galvanometer coil and the deflection of 

 the needle. 



Now, since Joule found the constant 



Htan6> = 0*17676 Vw, 



while the galvanometer coil, when measured in several places with a 

 standard foot rule, had a mean radius 



r = 0-62723 ft., 



, 0-62723 x 0-17676 

 the current c = 2x g. M16 



or c= 0-017645 </w. ■ 



The units are grains and feet ; on conversion to grams and centi- 

 meters c = 0-9742 x/iv amperes, that is to say, the constant of the 

 balance is 0*9742 . 



In the case of the newer balance, Joule found the constant by the 

 same method to be 6346. 



On referring back to the examination of the current balance, it will 

 be seen that for a current of about 3 amperes or 3'5 amperes, this 

 being the current at which Joule generally worked in these experi- 



2 b 2 



