Measurement of Small Capacities and Inductances. 495 



the function of which is to charge the capacity or condenser 

 to a given voltage and then discharge it through a galvano- 

 meter, repeating this process four times in each revolution of 

 the motor. This commutator is fixed on a shaft carried in well 

 lubricated bearings supported on two small A frames (PL XII. 

 tig. 1). On this shaft are held, by means of ebonite bushes 

 and washers, three gunmetal disks or wheels, of which the 

 centre one is in shape like an eight-rayed star, whilst the two 

 outer ones are like crown wheels, each having four teeth. 

 The three wheels are so set on the shaft that the teeth or 

 projections of each of the two outer wheels interlock or fall 

 in the space between the teeth of the other, whilst the radial 

 teeth of the intermediate wheel occupy the intervals between 

 the teeth of the two outer wheels. The developed surface of 

 this triple wheel is shown in fig. 2. The whole outer surface 

 is turned true and forms a barrel about four inches in 

 diameter, and two and a half inches wide. On this barrel 

 rest three brass gauze brushes which are carried in well 

 insulated brush holders, and by means of three springs and 

 levers the brushes are firmly pressed against the barrel, the 

 two outer brushes resting on the continuous portions or 

 flanges of the two outer wheels A and B, and the middle 

 brush occupying the centre line and making contact either 

 with the wheel A or wheel B, or with the intermediate 

 wheel I according to their position. It will be seen then 

 that as the commutator runs round, the middle brush is 

 alternately brought into metallic connexion with first one and 

 then the other of the two brushes on either side. The 

 function of the middle wheel (I) is to afford a stepping piece 

 and prevent any shock or jar as the middle brush passes over 

 from one connexion to the other. It also prevents the middle 

 brush from short-circuiting the two outer brushes at any 

 time. If then one terminal of the galvanometer is connected 

 to the brush pressing against the wheel A, and one terminal 

 of a battery is connected to the brush pressing against wheel 

 B, and one terminal of a condenser is connected to the middle 

 brush, the other terminals o£ the battery, galvanometer, and 

 condenser being connected together, it will easily be seen 

 that as the commutator runs round, the condenser is first 

 charged at the battery, and then discharged through the 

 galvanometer. In the following experiments, speeds of 1200 

 to 1700 revolutions per minute were used. To count the 

 rotations of the commutator a worm en the shaft drives a 

 wheel of such gear that the latter makes one revolution for 

 every hundred revolutions of the commutator. This wheel 

 carries a. pin which at each revolution causes a hammer to 



2 I, 2 



