PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 113 



these areas directly. As will appear in the sequel, it is possible, 

 though not very easy, to get good results in this way, even if the 

 cross-section of the laminated core is as great as, say, 800 square 

 centimeters ; for this, however, a properly constructed galvanometer is 

 required. 



The "time constant" of a circuit in which a current of given final 

 intensity is to be established is shorter the higher the electromotive 

 force used to generate the current ; it is desirable, therefore, to employ 

 a battery of rather high voltage and to reduce the current by non- 

 inductively wound resistance in series with the exciting coil of the 

 electromagnet. If a moving coil galvanometer is used, it is often neces- 

 sary to correct for the effect of the counter electromotive force induced 

 in the coil as it swings in the field of its own permanent magnet, and 



Figure 



A portion of the record of an oscillograph in the circuit of a secondary coil 

 wound on the core of an electromagnet when the current in the exciting coil is 

 made to change by sudden steps in the determination of a hysteresis cycle. 



it is always necessary to use steps so short and to make the period of 

 the galvanometer so long (perhaps 300 or 500 seconds) that the practical 

 duration of the induced current may be small in comparison. It is usual 

 to send the current to the exciting coil by means of a commutator and 

 a long series of manganine resistance coils capable of carrying the de- 

 sired currents ; these coils are often mounted in a frame furnished 

 with some device by which any or all of them can be shunted out of the 

 circuit at pleasure. Two rheostats, made for this purpose some years 

 ago by the Simplex Electric Company, have been found by the staff of 

 the Jefferson Physical Laboratory very satisfactory in practice. By 

 means of such a set of coils as those just described, one may easily get 

 either a progressive, step-by-step increase or decrease in the current, 

 or a reiteration of any particular step. One convenient way of arrang- 

 ing the apparatus for the repetition at pleasure of any desired step 

 has been recently described by A. H. Taylor. 8 The method of rever- 



8 A. Hoyt Taylor, Phys. Rev., 23, 1006. Mordey and Hansard, Elect. En- 

 gineer, 34, 1904. Searle and Bedford, Phil. Trans., 198, 1902. Drysdale, Jour. 

 Inst. Elect. Engineers, 31, 1901. Lamb and Walker, Electrical Review, 48, 1901. 



vol. xliii. — 8 



