346 Mr. J. D. Chorlton. Examination of some of the [Mar. 19, 



current meter which. Jo ale used in his determination of the equiva- 

 lent of heat by the electrical method, and it would be interesting 

 either to confirm the value which Joule gives, or to apply some 

 correction to it in case the constant were found to be rather different 

 from that assumed by him. 



I found two current meters at Sale, one of them being the meter 

 used in the determination of the equivalent of heat from the thermal 

 effects of electric currents . This meter is contained in an oblong 

 wooden box, 4 ft. in length and about 18 in. in height and width, 

 the balance beam is made of wood, and is suspended from the 

 sides of the box by stretched wires which, after passing along each 

 side of the beam, suspend a flat horizontal coil of wire 1 ft. in 

 diameter; above and below this coil are placed two fixed coils 

 exactly similar to the suspended one. The current is passed through 

 the coils in such a way that the top coil attracts while the lower 

 repels the movable one, electric communication with which is in- 

 geniously made by means of the suspending wires. 



At the other end of the beam is a counterpoise which brings 

 the movable coil into a position about midway between the two 

 fixed coils when no current is passing. When an electric current 

 is passed the suspended coil is attracted upwards and is brought 

 back into its former position by means of weights placed on a shelf 

 attached to it. 



Joule says of this balance : " The strength of a current can in 

 this manner be determined in absolute measure ; for the area of 

 each of the three coils being called a, the weight required to 

 counterpoise the force with which the suspended one is urged w, 

 the force of gravity g, and the length of wire in each of the coils I, 

 the current 



c = l/2l a / (l + correction), 



V 27T 



the correction being principally due to the distance between the 

 fixed coils. In my instrument, in which this distance is 1 in., the 

 diameter of the coils being 12 in. and their interior core 4 in., this 

 correction was proved by experiment to be 0*1185" ('Collected 

 Papers,' vol. 1, p. 543). 



Owing, however, to the difficulty in obtaining an exact measure 

 of the distance between the fixed coils, Joule abandoned this result 

 and found the constant of the instrument by comparing it with a 

 tangent galvanometer. 



Tn order to re-determine this constant, I passed a current through 

 this balance and through one of Lord Kelvin's current balances in 

 the Physical Laboratory at Owens College placed in series with it. 



Thus, if w be the weight necessary to bring back the movable coil 



