Upon the completion of the winding of a coil, regardless of what method is used, it is customary to place a layer of bookbinders’ cloth over the coil so as to afford a certain mechanical protection for the insulated wire.

Winding Terminals.  The matter of bringing out the terminal ends of the winding is one that has received a great deal of attention in the construction of electromagnets and coils for various purposes.  Where the winding is of fine wire, it is always well to reinforce its ends by a short piece of larger wire.  Where this is done the larger wire is given several turns around the body of the coil, so that the finer wire with which it connects may be relieved of all strain which may be exerted upon it from the protruding ends of the wire.  Great care is necessary in the bringing out of the inner terminal—­i.e., the terminal which connects with the inner layer—­that the terminal wire shall not come in contact with any of the subsequent layers that are wound on.

[Illustration Fig. 100.  Electromagnet with Terminals]

Where economy of space is necessary, a convenient method of terminating the winding of the coil consists in fastening rigid terminals to the spool head.  This, in the case of a fiber spool head, may be done by driving heavy metal terminals into the fiber.  The connections of the two wires leading from the winding are then made with these heavy rigid terminals by means of solder.  A coil having such terminals is shown in its finished condition in Fig. 100.

Winding Data.  The two things principally affecting the manufacture of electromagnets for telephone purposes are the number of turns in a winding and the resistance of the wound wire.  The latter governs the amount of current which may flow through the coil with a given difference of potential at its end, while the former control the amount of magnetism produced in the core by the current flowing.  While a coil is being wound, it is a simple matter to count the turns by any simple form of revolution counter.  When the coil has been completed it is a simple matter to measure its resistance.  But it is not so simple to determine in advance how many turns of a given size wire may be placed on a given spool, and still less simple to know what the resistance of the wire on that spool will be when the desired turns shall have been wound.

TABLE IV

Winding Data for Insulated Wires—­Silk and Cotton Covering

A.W.G.  B & S  |   20       21       22       23        24       25
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DIAMETER      |
Mils          |   31.961   28.462   25.347   22.571   20.100   17.900
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AREA          |
Circular Mils | 1021.20   810.10   642.70   509.45   404.01   320.40
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