166 Prof. J. Trowbridge on Methods of Measuring 



No. 3. Heat Method. 

 By trie use of the law that the heat developed in a circuit 

 is expressed by H = C 2 Ri, where C is current in Vebers, R = 

 resistance, t = time, we can deduce C 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 conditions for a 

 given experiment. 



No. 4. The Electrodynamometer Method. 



The principle of Weber's electrodynamometer is well known. 

 The electric current passes down one wire of the bifilar sus- 

 pension of a movable coil and up the other, and then through 

 fixed coils surrounding the movable coil. Maxwell, in his 

 c Electricity and Magnetism,' vol. ii. p. 332, remarks : — 

 a Weber's form of the electrodynamometer, in which one coil 

 is suspended within another, and is acted on by a couple tend- 

 ing to turn it about a vertical axis, is probably the best fitted 

 for absolute measurements." With powerful currents, how- 

 ever, 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 circuit is generally of the same order of mag- 

 nitude 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 driving the dynamo-electric 

 machine, the velocity of the machine, and the resistance of the 

 circuit. It is also important to eliminate local attractions. 

 The time consumed in measuring the current-strength should 

 be small. 



The instrument described in this paper fulfilled the condi- 

 tions prescribed. 



Fig. 1 (PI. Till.) shows the general aspect of the apparatus. 

 The large fixed coils were made of copper bands, 35 millims. 

 broad and 1 millim. thick. There were twelve coils, six on 

 each side of the movable coil, which is shown with its suspen- 

 sion between them. The large coils we reinsulated from each 

 other by vulcanite washers, and held together by brass rivets 

 insulated by vulcanite cylinders. The coils were placed at a 

 distance apart equal to their thickness, and thus allowed cur- 



