regardless of the actual mechanical details of the particular hook
switch which happened to be employed. Thus diagram _A_ in Fig. 88
shows a hook switch having a single make contact and this diagram
might be used to refer to the hook switch of the Dean Electric Company
shown in Fig. 85, in which only a single contact is made when the
receiver is removed, and none is made when it is on the hook.
Similarly, diagram _B_ might be used to represent the hook switch of
the Kellogg Company, shown in Fig. 83, the arrangement being for two
make and two break contacts. Likewise diagram _C_ might be used to
represent the hook switch of the Western Electric Company, shown in
Fig. 84, which, as before stated, has two make contacts only. Diagram
_D_ shows another modification in which contacts made by the hook
switch, when the receiver is removed, control two separate circuits.
Assuming that the solid black portion represents insulation, it is
obvious that the contacts are divided into two groups, one insulated
from the other.

[Illustration: Fig. 88. Hook Switch Symbols]

[Illustration: COMPRESSED AIR WAGON FOR PNEUMATIC DRILLING AND
CHIPPING IN MANHOLES]




CHAPTER X

ELECTROMAGNETS AND INDUCTIVE COILS