VIII. On the measurement of the magnetic properties of iron

Abstract

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 Lose 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 has been usual to determine by means of a series of successive experiments the value of the total magnetization pft)duced 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 ∫ n 2 n 1 L de , or its equivalent ∫ l2 l1 e dt can bre measured. In the first form of the integral, c 1 and c 2 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 2 — t 1 is the interval of time required for the current to change from the value c 1 to the value c 2 , and e is the back electromotive force induced by the rate of change of the current at any instant between the times t 1 and t 2 . When the masses of iron experimented on are large, the interval of time t 3 — t 2 becomes too great for the value of the integral to be accurately measured by means of any ordinary form of ballistic galvanometer joined in circuit with a second, or induction, coil surrounding the iron.