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.

+==========+
|=|                |=|      |=|           |=|      |=|
\=======           \=======  =======/      =======  =======
|     |            |     |  |     |       |     |  |     |
|     |            |     |  |     |       |     |  |     |
|     |            |     |--|     |       |     |  |     |
=======            =======  =======      /=======  =======\
|=|                |=|      |=|           |=|      |=|

+===========+            +==========+            +===== ======+
|=|      |=|           |=|      |=|            |=|        |=|
=======  =======/    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