Thus, by fixing them at the extremity of the
branches the number of the vibrations is reduced to 5 or 6.
There will be seen in the figure the electro-magnet which keeps up the
vibration. This is formed of three simple electro-magnets, whose bobbins
have a resistance of no more than 10 ohms, and which are united in
series. The interrupting plate, P, against which the style, s, rests
at each vibration, is capable of a forward movement, or one of recoil,
by the aid of a screw, V, and of an eccentric movement which is produced
by a small handle, m, and during which its plane remains invariable.
This arrangement permits the point of contact of the style and plate to
be varied without changing the precision with which the contact takes
place, and all the points of the plate to be slowly used in succession
before replacing it. The motion is produced by means of a relatively
weak pile, whose poles are connected to the terminals, A and A'. Three
Callaud elements of triple surface, renewed one after the other every
month at the most, are sufficient to keep up the vibrations
continuously, day and night, without interruption, and that too even
when the instrument is employed in producing a small mechanical power,
as we shall see further on.
We have now seen how electro-tuning forks may be constructed of large
dimensions, of large mass, and giving a small number of vibrations per
second.
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