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On the Measurement of the Magnetic Properties of Iron. 
By Thomas Gray, B.Sc., F.R.S.E. 
Communicated hy Lord Kelvin, P.R.S. 
Received May 3,—Read May 19, 1892. 
[Plates 5-16.] 
This paper contains some of the results and a description of the methods employed in 
a series of experiments on the rate of change of an electric current, immediately after 
the application or reversal of a constant electromotive force, in a circuit containing 
the magnetizing coil of a large electromagnet or the primary coil of a transformer. 
The experiments have been carried out in the Electrical Laboratories of the Rose 
Polytechnic Institute, Terre Haute, Ind. The results seem to be of considerable 
interest, and the method of studying the magnetic properties of iron here proposed 
has, I believe, some important advantages in many practical cases where the magnetic 
circuit is closed and of large section. 
In the experimental determination of the magnetic properties of iron it lias been 
usual to determine by means of a series of successive experiments the value of the 
total magnetization produced by different magnetizing forces. From these results the 
magnetic permeability of the iron, the self-induction of the circuit, and so forth, can, 
of course be calculated. Several methods are well known, by means of which reliable 
results can be obtained in this way, but they are, in many cases, inconvenient. For 
closed magnetic circuits, for example, the method commonly employed has been to 
measure, by means of the current induced in a coil of wire surrounding the iron, and 
in circuit with a ballistic galvanometer, the changes of magnetization produced by 
different changes of the current in a magnetizing coil. By this method, the value of 
the integral | L dc, or its equivalent | e dt, can be measured. In the first form of 
the integral, and are the initial and final values of the current in, and L the 
coefficient of induction of, the magnetizing coil. In the second form, t^ — b 
interval of time required for the current to change from the value to the value c^, 
and e is the back electromotive force induced by the rate of change of the current at 
any instant between the times q and t^. When the masses of iron experimented on 
are large, the interval of time t^ — G becomes too great for the value of the integral 
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