ELECTRICAL MEASUREMENTS 315 



The Westinghouse dynamo was one having 10 poles so that each revo- 

 lution of the armature produced 5 complete periods. The period of 

 this dynamo was determined by taking the time of 1000 revolutions of 

 the armature. This was accomplished by having the armature make 

 an electric connection with a bell every 200 revolutions and taking the 

 time of 5 of these. The taking of the speed during every experiment 

 gave more regular results, as the speed was constantly changing, the 

 dynamo being run by the engine in the University power-house when it 

 was subject to great change of load. This dynamo had a period of 

 about 132 complete periods per second. 



For the production of a current of less period than that of the West- 

 inghouse, the small alternator constructed at the University was used. 

 This dynamo was run by a small continuous Sprague motor. The arma- 

 ture of the small alternator consisted of 8 coils, which coils were fas- 

 tened flat on a German silver plate, the plate revolving between 8 field 

 pieces producing 4 poles. The object of having the coils of the arma- 

 ture on a metal plate was to secure a nearly constant speed. The metal 

 plate produced a load that varied as the velocity and due to induced 

 currents in the plate. The varying load, depending on the velocity of 

 the moving plate, produced a nearly constant speed, which rendered 

 unnecessary the constant taking of the speed. When this dynamo was 

 used, the speed was only determined two or three times during a series 

 of readings or experiments. The average of these determinations was 

 taken as the speed during the whole series of experiments under con- 

 sideration. 



Coils. The coils whose inductances were determined were all made 

 in the same way, being wound on a metal form and soaked in a prepa- 

 ration of wax. When the wax was hard the metal form was removed. 

 This enabled the coils to be placed close together, as their sides were 

 flat and smooth. The coils all had the same internal and external 

 diameter, but their width varied, that being determined by the number 

 of turns that were desired. 



Coils. P v External diameter 35-46 cm., internal diameter 23-8 

 cm., was made up of about 1200 turns of No. 16 B and 8 gauge single 

 covered cotton copper wire, roughly wound; the turns were not smooth; 

 self-inductance as finally determined -566 henry. 



P 2 ., Same dimensions. Turns were put on evenly. The number 

 of turns was 1300 of No. 16 B and 8 single covered cotton copper wire. 

 Self-inductance -724 henry. 



A. Same internal and external diameters as P, but the width was 



