294 BELL SYSTEM TECHNICAL JOURNAL 



5. The Power Unit. 



The alternating magnetizing current is derived from a low speed 

 dynamotor. The machine operates on 24 volt storage battery power, 

 and delivers at low load, nearly sinusoidal current of 24 volts amplitude. 

 Resistance in the field circuit permits regulation of the frequency 

 between 10 and 30 cycles, while rheostats in each side of the output 

 of the machine serve to regulate the current for the magnetizing coil. 



Mounted in the power unit and connected directly across its output 

 terminals there is an electrolytic condenser of about 1,200 mf. {E of 

 Figs. 1 and 2). This condenser serves three important functions: 

 a. It smoothes out ripples in the magnetizing current which would, 

 if they were large enough, produce secondary loops in the hysteresis 

 curve, h. It makes the power unit a source of potential having low 

 impedance to a.c. so that the magnetizing current is in part determined 

 at any moment by the counter e.m.f. of the magnetizing coil, rather 

 than wholly by external impedances. This being so, the magnetizing 

 current is retarded in the steep parts of the hysteresis curve of the 

 sample where the counter e.m.f. is great. Eddy currents do not, 

 therefore, build up in the sample nearly so much when the condenser 

 is in the circuit as when it is not. c. The spot on the oscillograph 

 tube being thus slowed down on the steep parts of the hysteresis 

 curve and correspondingly speeded up on the saturation parts results 

 in a much more uniform brightness of pattern which can be observed 

 more readily. 



6. Magnetizing Coils, Search Coils, and Samples. 



The samples which have been used are mostly in the form of either 

 flat rings stamped from sheet metal, or long straight strips of thin tape. 

 The rings are about four inches in diameter so as to fit into a toroidal 

 furnace made for magnetic testing.'* When the furnace is used the 

 windings are applied on the furnace, 45 turns for the magnetizing 

 winding and 90 turns for the search coil. When used without the 

 furnace a similar number of turns are wound directly on the sample. 

 The number of turns in the search coil being small the cross-sectional 

 area of the sample is made correspondingly large, about 1/4 square 

 inch in order to apply sufficient voltage to the integrator. 



The apparatus for use with the single ribbon samples consists of 

 straight magnetizing and search coils. A glass tube about \\ inches 

 in diameter carries two parallel windings 30 inches long, for producing 

 the magnetizing field. At the center of the length of this coil and 



* The furnace will be described by Mr. G. A. Kelsall in a forthcoming number of 

 the Journal of the Optical Society of America and Review of Scientific Instruments. 



