8 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



figures of merit, values for saturation fiux, ip" , and leakage fiux ratios at 

 various points in the magnetic circuit, are commonly needed. For such 

 information, flux as a function of ampere turns must be measured. Per- 

 haps the most effective method is to make determinations of the average 

 flux in the magnet structure at a numl^er of different air gaps; then by 

 analysis determine values for the constants just enumerated. A series of 

 magnetization curves for the relay magnet are required similar to those 

 commonly taken on magnetic "ring samples," except that curves are 

 obtained for several different air gaps. The ballistic galvanometer is the 

 most familiar instrument for this purpose, still used for certain problems. 

 In the Bell Laboratories, however, an extremely versatile recording 

 fluxmeter was developed some ten years ago by P. P. Cioffi for use in 

 research on magnetic materials.'' One of these instruments is now in con- 

 stant use for relay measurements because of its accuracy, versatility and 

 the rapidity with which tests can be made. The complete unit is shown 

 in Fig. 3, being made up of a power supply system, galvanometer-integra- 

 tor unit, and X-Y recorder. Its operation will now be described very 

 briefl}'. 



The magnet to be tested is provided with a search coil. This search 

 coil may be in the form of a winding physically in parallel with the 

 main supply winding, a coil of few turns uniformly spread over the out- 

 side of the coil, or of a specially wound coil mounted at a particular 

 point of interest. When current is progressively ^'aried through the main 

 winding, the voltage induced in the search coil by the magnet flux is 

 transmitted to the galvanometer whose associated optical system divides 

 a light beam between two photocells. The resulting photocell voltage 

 unbalance is amplified and coupled back into the search coil circuit 

 through a mutual inductance, so poled as to tend to restore the gal- 

 vanometer deflection to zero. The feedback current in the mutual circuit 

 is proportional to the flux, and is used to dri^■e the pen on the X-Y 

 recorder. The paper is driven proportionately to the current in the wind- 

 ing, wath the result that flux versus current (or ampere turns) is plotted 

 directly. The instrument gives accuracy of i^a per cent over a wide 

 range of fluxes and currents, with provisions made for readily changing 

 the scale to cover different windings, operating voltages or magnet 

 shapes. Measurements are made in a few minutes compared to a con- 

 siderably greater effort when using the ballistic gah'anometer. Auxiliary 

 features are also provided permitting convenient and complete demag- 

 netization of the test magnets betw'een readings. Readings are extremely 

 stable and repeatable. Typical results are shown in Fig. 4 where a series 

 of magnetization curves for a particular test magnet are shown. By 



