itch spring itself serves
by its own strength to raise the hook lever when released from the
weight of the receiver.

[Illustration: Fig. 87. Desk-Stand Hook Switch]

In this switch, the hook lever, and in fact the entire exposed metal
portions of the instrument, are insulated from all of the contact
springs and, therefore, there is little liability of shocks on the
part of the person using the instrument.

Conventional Symbols. The hook switch plays a very important part
in the operation of telephone circuits; for this reason readily
understood conventional symbols, by which they may be conveniently
represented in drawings of circuits, are desirable. In Fig. 88 are
shown several symbols such as would apply to almost any circuit,
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

Electromagnet. The physical thing which we call an electromagnet,
consisting of a coil or helix of wire, the turns of which are
insulated from each other, and within which is usually included an
iron core, is by far the most useful of all the so-called translating
devices employed in telephony. In performing the ordinary functions of
an electromagnet it translates the energy of an electrical current
into the energy of mechanical motion. An almost equally important
function is the converse o