22 



BELL SYSTEM TECIIXICAL JOURNAL 



for changing ihe magnetic lield in the specimen, is sIkjwii in I^'ig. 2. A 

 drum D, carrying photographic paper, is placed in a Hght-tight box 

 provided with a long, narrow slit parallel to the axis of rotation of the 

 drum. A beam of light from a second lamp is reflected by the fluxmeter 

 mirror and focused on the slit. This beam is reflected by the same 

 mirror which reflects the beam onto the photo-electric cell, the two 



X 



Fig. 2 — The field current circuit and the piiotographic drum. 



beams being incident at different angles. Attached to the shaft of 

 the drum is an arm A, which slides along the rheostat R. A battery B 

 is connected across R, and a center tap soldered to it. Between the 

 arm A and the center tap a varying e.m.f. is produced which is applied 

 to the held coil F. This e.m.f. reverses its sign e\ery time the arm A 

 slides past the center of the rheostat, and the latter is curved in a 

 manner calculated so that the held current will be jjroportional to the 

 angle of rotation of the drum from the position for zero current. The 

 .search coil 6" of Fig. 1 is placed within F, and consequently when D is 

 rotated it moves the photographic paper past the slit so that the 

 distance moved is proportional to the change in held current, while at 

 the same time the fluxmeter deflects the beam of light along the slit so 

 that the deflection is proportional to the time integral of the changes 

 of flux within S. As the drum is turned from one position to another, 

 a curve with rectangular axes is thus registered, the scales of which 

 may be calibrated in terms of B and //. Figs. 4 to 7 are some examples 

 of curves taken with the apparatus. 



In Fig. ?) the electrical circuits are shown in detail. R^ is the rheostat 

 controlling the held current, and A is the arm which rotates with the 



