L, in quadrants. Bobbins, separately, without iron cores. 0.233 and 0.265 Bobbins, separately, with iron cores. 1.65 and 1.71 Bobbins, with cores joined by yoke, coils in series 6.37 Bobbins, with armature resting on poles. 10.68 [Footnote 1: "Bulletin de la Societe Internationale des Electriciens," 1886.] It is interesting to note how the perfecting of the magnetic circuit increases the self-induction. Thanks to the kindness of Mr. Preece, I have been furnished with some most valuable information about the coefficients of self-induction, and the resistance of the standard pattern of relays, and other instruments which are used in the British postal telegraph service, from which data one is able to say exactly what the time constants of those instruments will be on a given circuit, and how long in their case the current will take to rise to any given fraction of its final value. Here let me refer to a very capital paper by Mr. Preece in an old number of the "Journal of the Society of Telegraph Engineers," a paper "On Shunts," in which he treats this question, not as perfectly as it could now be treated with the fuller knowledge we have in 1890 about the coefficients of self-induction, but in a very useful and practical way. He showed most completely that the more perfect the magnetic circuit is--though of course you are getting more magnetism from your current--the more is that current retarded. Mr. Preece'e mode of experiment was extremely simple. 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. +==========+ |=| |=| |=| |=| |=| \======= \======= =======/ ======= ======= | | | | | | | | | | | | | | | | | | | | | | | |--| | | | | | ======= ======= ======= /======= =======\ |=| |=| |=| |=| |=|