Electro-Motive Force and Current Curves. 153 



taken from the other brush and the other pole of the 

 machine to an electrometer, the electrometer becomes 

 connected to the poles of the dynamo, at one definite point 

 in the revolution of the armature. This point is known, 

 and can be altered by the arm which carries the two insu- 

 lated brushes moving round a fixed divided circle. 



Method of obtaining Current Curves. — This arrangement 

 would give the E.M.F. curves at the terminals of the machine. 

 To obtain the current curves, the fact is made use of that 

 the current in a non-inductive resistance is in phase with, 

 and proportional to, the E.M.F. at the terminals of the 

 resistance. Therefore, if the electrometer can be connected 

 through the same intermittent contact apparatus to the 

 terminals of an ohmic resistance, the form of the current 

 curve can be obtained. 



The Electrometer. — The Electrometer used was Mascart's 

 quadrant, and consisted of an aluminium needle suspended 

 in the quadrant by a silver wire ; to the needle was attached 

 the mirror and a damping vane dipping into oil. Each 

 opposite pair of quadrants was connected to the ends of a 

 water battery of 48, 96, or 144 cells ; the middle of the 

 battery was always connected to the frame of the instrument. 

 The wires from the intermittent contact apparatus were 

 connected respectively to the frame, and to the needle by 

 means of the silver wire suspension. 



With this arrangement of the electrometer, the deflection 

 is directly proportional to the difference of potential between 

 the needle and the frame. 



The constant of the instrument was found by means of a 

 battery of Clark's cells. ; ; 



With 24 water cells I scale div. = 'Sy volts. 

 » 48 » » ='43 »\ 



Magnetization Curves of Field Magnets. — To obtain the 

 magnetization curve of the iron of the field magnets, a flat 

 coil of 12 turns' of wire was wound, and arranged so that it 



