THE MAGNETIC PROPERTIES OF IRON. 
535 
cores. Thus, bj turning the screw, the cores can be moved apart or brought closer 
together. A similar arrangement serves to move the pole pieces and to pull them 
apart after they have been strongly magnetized on closed circuit. When an air 
space is left between the pole faces, the magnetic attraction is resisted by distance 
pieces, placed between the poles. The necessity for the soniewliat elaborate arrange¬ 
ment just described arises from the great weight of the pieces, the cross section of 
the iron circuit being about 50 square inches, and the length of the yoke 40 inches. 
The cores and pole pieces were constructed of soft homogeneous iron, laminated in 
sheets one-twentieth of an inch thick. Each part is held together by plates three- 
eighths of an inch thick, bolted together through the sheets. One plate of eacli piece 
forms a common nut for all the bolts, while the heads of the bolts are countersunk 
into the other plate. The insulation between the sheets and between the bolts and the 
holes is simply the oxidized surface of the rolled metal coated with shellac varnish. 
Let the impressed electromotive force be E, the resistance of the circuit R and the 
current at any instant C. Then E — PtC = e, tlie back electromotive force on the 
magnetizing coil due to induction. This is also equal to L idCldt), and hence we have 
e = E — RC = L {dQjdt) or edt = dG. Consider the curve Ohd in the diagram. 
Fift-. 3. 
fig. 3, to be a curve showing the relation between current and time immediately after 
the magnetizing circuit is closed. Let the distance along OX to any point represent 
the interval of time — C and the vertical ordinates the successive values of RC. 
Then the maximum vertical ordinate Oc = E, and the line ah corresponding to any 
time T is the value of e at (diat time. Hence the area Ohac is the value of the integral 
ecZ^orof j Lc/C, the total change of induction from the time of closing the circuit 
to the time T; the current being supposed zero when T is zero. By means of this 
curve the total induction produced by any current between zero and the maximum 
that the impressed E.M.F. is capable of producing can thus be studied. Again, by 
drawing tangents to the curve at points corresponding to diiferent values of the 
currents flowing through the circuit, the value of dQjdt and, consequently, of 
