552 Prof. T. R. Lyle ; Prelitniaary Account of 



four-pole alternator if attached direct to it, or for a four-pole 

 synchronous motor. 



A and B are called the fixed brushes, though exigencies of 

 construction render it necessary to attach one of them at least 

 to the movable divided circle. Each has permanent contact, 

 however, always with the same section of the commutator. 

 C and D are the movable brushes whose points of contact 

 are 90° (equivalent to 180° in phase) apart, and are attached 

 to a divided circle which can be rotated by means of a tangent- 

 screw. 



For any definite position of C and D, given by the reading 

 of the circle against a fixed index, commutation takes place 

 at the corresponding point of the wave, and the steady galva- 

 nometer deflexion multiplied by the constant in § 3 gives the 

 instantaneous value of the primary current at that point of 

 the wave. 



Readings of the galvanometer having been taken at suitable 

 intervals over one half turn of the divided circle, these can 

 be plotted against the circle readings multiplied by 2, and a 

 complete wave-form is obtained. 



It is essential that the large resistance r should be on 

 the galvanometer side of the commutator. The latter will 

 nearly always act on its own account as a feeble gene- 

 rator of statical electricity, and if the circuit be broken 

 on the M side, the galvanometer will give small deflexions 

 which vary in magnitude and sign with the position of the 

 movable brushes. These deflexions are independent of the 

 magnitude of r, and are greatly increased by approaching 

 an excited rod of ebonite to the commutator. When the 

 connexions are as described, these statical discharges are 

 short-circuited through the very low resistance of M and 

 cause no trouble. 



5. In order to photograph wave-forms the galvanometer is 

 inclosed at one end of a long camera-box, at the other end of 

 which a dark slide containing a photographic plate moves in 

 vertical grooves behind a metal screen, across the middle of 

 which is cut a narrow horizontal slit. The slide is suspended 

 by a vertical cord or fine wire, which is led through a hole 

 in the box, thence over a pulley to a groove cut round the 

 divided circle that carries the movable brushes. Light from 

 an incandescent lamp enters throuoh a vertical slit placed 

 alongside the dark slide, and is reflected by the galvanometer- 

 mirror to the horizontal slit behind which the photographic 

 plate moves. Each plate can be standardized by means of a 

 Clark's cell and megohm, or by the Kelvin -balance method 

 in § 3. 



