152 Laby and Roberts: 



bearing of the stator, the rotor was balanced. Suitably illuminated 

 points situated on the plate in question were observed with a 

 microscope, carried by a support free from vibration, and the 

 rotor was balanced until tlie amplitude (i.e., diameter of circum- 

 scribing circle) of the vibration of the plate was reduced to .001 

 cm/. 



The^ field magnets have two poles, and there are two windings on 

 each pole. These windings can be connected in series so that the 

 turns reinforce one another, and the maximum flux for a given 

 current produced. They are so connected for the heavy experiment. 

 The windings can also be connected in series so as to oppose one 

 another in their magnetising effect. They are so connected for the 

 light experiment. In this way the flux can be reduced to one-tenth 

 without changing the current in the windings and, therefore, with- 

 out changing the temperature of the rotor in the light and heavy 

 experiments. 



77i<? Stator and Calorbneter. — The construction of the stator i& 

 shown in Figs. 5, 6 and 7. Fig. 5 shows copper and iron cylinders. 

 The iron cylinder increases the magnetic flux, and supports the 

 copper cylinder. The channels on the external surface of the iron, 

 and the axial hole, carry the water in its circulation through the 

 apparatus. The iron cylinder is attached to a glass tube (see Fig. 

 6) filled with eider down, in order to reduce loss of heat by con- 

 duction. The glassy tube in turn is attached to a steel shaft which 

 passes through a ball-bearing, and, at its upper end, is suspended 

 by a steel torsion wire. The wire supports the whole weight of the 

 shaft and calorimeter. 



A thin sheet cylinder encloses the copper cylinder, and a Dewar 

 flask encloses the steel, copper, and iron cylinders, Avhich to pre- 

 vent corrosion are all silver plated; 



The water enters the calorimeter (see Figs. 5, 6 and 7) through 

 a rubber tube, flows doAvnwards between the inside wall of the 

 Dewar flask and the thin steel cylinder, turns upwards and flows be- 

 tween the steel and the copper, then down between the copper and 

 iron, and finally out of the calorimeter through the axial hole in the 

 iron armature. The object of this somewhat elaborate circulation 

 is — (1) to bring the water into thorough contact with the copper 

 and iron in which the heat is generated, (2) to break up stream 

 lines, and so ensure that the inlet and outlet water temperatures 

 determine the heat losses of the calorimeter. 



The ball bearing which maintains the shaft vertical was very 

 carefully made, and is used without oil lubrication, which was 

 found to increase the friction. The angular amplitude of the stator 



