+===========+ +==========+ +===== ======+ |=| |=| |=| |=| |=| |=| ======= =======/ B\======= =======/A A\======= =======/B | | | | | | | | | | | | | | | | | | | | | | | | | |--| | | | | | | | | | ======= ======= A======= =======B =======B =======A |=| |=| |=| |=| |=| |=| +==========+ +==========+ +====== =====+ 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 expected. Lastly, when the armature and yoke were both of them split in the middle, as is done in fact in all the standard patterns of the British postal telegraph relays, the throw of the galvanometer was brought down from 502 to 26. Relays so constructed will work excessively rapidly. Mr. Preece states that with the old pattern of relay having so much self-induction as to give a galvanometer throw of 1,688, the speed of signaling was only from 50 to 60 words per minute, whereas, with the standard relays constructed on the new plan, the speed of signaling is from 400 to 450 words per minute. It is a very interesting and beautiful result to arrive at from the experimental study of these magnetic circuits. SHORT CORES _versus_ LONG CORES.