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296 RICHARDSON'S BLUEBOOK OF PROJECTION
high brush loss, though too much brush pressure tends to wear both brush and metal surface unduly, and to cut grooves in the surface of the metal.
Brush Pressure
(63) Brush pressure refers to the pressure by which brushes are held to the contact surface — by means of a spring. It is considered in terms of the pressure exerted by each square inch of brush or contact surface. (64) The pressure must be just sufficient to insure good electrical contact when the surfaces are smooth and clean; it should never be less than 1 34 or more than 1 ]/2 pounds per square inch of contact surface. (65) Too much pressure means excessive brush wear; too little pressure induces sparking, roughening of the metal surface and a bad condition all around. (66) The pressure may be measured roughly by means of a small, accurate spring scale. (Test the scale for accuracy by weighing something previously weighed on a standard scale.) Attach it to the end of each brush finger by any convenient means, and note the pull required to just start the finger off the brush. Then calculate the surface of the brush in square inches (width of brush multiplied by the width of its contact surface) and adjust the pressure so that it will not be less than \% or more than \y2 pounds per square inch.
Brush Rocker or "Yoke"
(67) This is a metallic yoke to which the brush holders are attached and held by it in proper position. This "yoke" may be moved (rocked) backward or forward to change the position of the brushes circumferentially with relation to the commutator. This brush position is highly important. Once located at the point of least possible sparking, means are provided to lock the yoke into position.
C ompound-zvound Generator
(68) Upon the field magnet of this machine are wound two sets of coils, one of which is connected in series and