OF ARTS AND SCIENCES. 123 



with static electricity in electrical measurements is well known. Leak- 

 age, want of constancy of charge in the electrometer, nay, impossi- 

 bility of maintaining a charge in certain localities, limit the use of 

 this method, even if the results obtained were not approximate. 



No. 3. Heat Method. 



By the use of the law that the heat developed in a circuit is ex- 

 pressed by H = C 2 Rt, where C is current in Vebers, R = resistance, 

 t = time, we can deduce G by measuring the rise of temperature of 

 a given volume of water. Measurements of temperature are especially 

 fraught with difficulties on account of conduction, radiation, and errors 

 of thermometers, beside consuming time in waiting for the proper con- 

 ditions for a given experiment. 



No. 4. 77ie Electro dynamometer Method. 



The principle of Webers' electrodynamometer is well known. The 

 electric current passes down one wire of the bifilar suspension of a 

 movable coil and up the other, and then through fixed coils surround- 

 ing the movable coil. Maxwell, in his " Electricity and Magnetism," 

 Vol. II. p. 332, remarks : " Webers' form of the electrodynamometer, 

 in which one coil is suspended within another, and is acted on by a 

 couple tending to turn it about a vertical axis, is probably the best 

 fitted for absolute measurements." With powerful currents, however, 

 it is necessary to shunt this instrument, and the errors inherent in this 

 method are introduced. Even with moderate currents, the directive 

 force of the bifilar suspension is changed by the elongation of the 

 wire from a rise in temperature. If we keep within the point at 

 which the wires are elongated, the deflections are slight and subject to 

 error of observation. 



In working with dynamo-electric machines, it is important that we 

 should avoid the method of shunts ; for the entire resistance of the cir- 

 cuit is generally of the same order of magnitude as the shunts employed. 

 It is necessary that we measure the whole strength of the current 

 directly at the same time that we measure the work consumed in driv- 

 ing the dynamo-electric machine, the velocity of the machine, and the 

 resistance of the circuit. It is also important to eliminate local attrac- 

 tions. The time consumed in measuring the current strength should 

 be small. 



The instrument described in this paper fulfilled the conditions 

 prescribed. 



Fig. 1 shows the general aspect of the apparatus. The large fixed 

 coils were made of copper bands, 35 mm. broad and 1 mm. thick. 



