mple. He observed the throw of the
galvanometer when the circuit which contained the battery and the
electromagnet was opened by a key which at the same moment connected
the electromagnet wires to the galvanometer. The throw of the
galvanometer was assumed to represent the extra current which flowed
out. Fig. 56 represents a few of the results of Mr. Preece's paper.

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FIG. 56.--ELECTROMAGNETS OF RELAY, AND THEIR EFFECTS.

Take from an ordinary relay a coil, with its iron core, half the
electromagnet, so to speak, without any yoke or armature. Connect it
up as described, and observe the throw given to the galvanometer. The
amount of throw obtained from the single coil was taken as unity, and
all others were compared with it. If you join up two such coils as
they are usually joined, in series, but without any iron yoke across
the cores, the throw was 17. Putting the iron yoke across the cores,
to constitute a horseshoe form, 496 was the throw; that is to say, the
tendency of this electromagnet to retard the current was 496 times as
great as that of the simple coil. But when an armature was put over
the top, the effect ran up to 2,238. By the mere device of putting the
coils in parallel, instead of in series, the 2,238 came down to 502, a
little less than the quarter value which would have been ex