1894] and Measuring Difference of Phase, fyc. 251 



occurrence of this contact can be adjusted to any instant of the 

 period T, the instant of passage of the alternating E.M.F. or current 

 through from positive to negative, or vice versa, can be accurately 

 determined. 



The contact need not be absolutely momentary if the needle of the 

 galvanometer have inertia sufficient, nor need it occur in each 

 period, provided that the recurring period be an exact multiple of 

 the alternating period. 



On the principles enunciated above, the following simple appa- 

 ~atus has been devised for the exact measurement of the angle 



A cylinder of boxwood or ebonite is caused to rotate synchron- 

 ously with the alternating current generator, making one revolution 

 to a complete period, either by direct connexion of its axle with that 

 of the machine through suitable multiplying gear, in a manner 

 similar to that used for the ordinary velocimeter, or by driving it by 

 a synchronising motor. 



In the surface of the cylinder is embedded one metal wire or strip 

 parallel with the axle, and connected to the axle or to a contact ring. 



Two insulated springs press against the surface of the cylinder ; 

 one of these, called the Volt brush V, is attached to a dial face 

 accurately marked with degrees, centred at the axis of revolution, 

 and capable of rotation round this axis, and provided with, clamping 

 and slow motion screws ; the other brush, called the current brush 

 C, is attached to an index moving over the face of the dial, and also 

 provided with clamping and slow motion screws. For very exact 

 measurement the index may carry a vernier. 



The brushes are so arranged that they make simultaneous contact 

 with the wire on the cylinder when the index is exactly at the 

 zero of the dial, and the cylinder is rotated. This is tested by means 

 of a battery and galvanometer, and the brushes are provided with 

 suitable means of adjustment. If the wire is of appreciable width, 

 the adjustment of the brushes should be such as to give maximum 

 deflection on the galvanometer. 



The connexions are as follows : One terminal of the alternator is 

 connected to the axle or contact ring of the cylinder by means of an 

 ordinary rubbing contact ; Brush V is connected to a sensitive dead- 

 beat galvanometer, which can be shunted at will, and coils of 

 sufficient impedance to enable the shunted galvanometer to withstand 

 the full E.M.F., and to the other terminal of the alternator ; Brush C 

 is connected through a resistance which can be cut out of circuit to 

 a low resistance galvanometer, and thence to a point on the main 

 connected with the axle of the cylinder at a short distance from this 

 connexion, so that a short piece of main is a shunt to this galva- 

 nometer and contact. 



