The coils of the electromagnets are differentially wound with silk-covered wire, 4 mils (= 004 inch) in diameter, to a total resistance of 400 ohms. This differential winding enables the instrument to be used for " duplex " working, but the connexions of the wires to the terminal screws are such that the relay can be used for ordinary single working.
Screw adjustments are provided for closing or opening the air gap between the electromagnets and armatures, for raising or lowering the siphon, and for adjusting the point of the siphon to the centre or side of the paper strip. The received signals are recorded on the paper strip in an undulating continuous line of ink, and are distinguished by the length of deviation from zero.
The amplitude of the signals can be varied in several ways, either by a shunt across the electromagnet, or by altering the tension of the controlling springs or by altering the air gap between electromagnets and armatures.
At regular intervals a rotating arm on the distributor connects the five keys of each keyboard to line, thus passing the signals to the distant station, where they pass through the distributor and certain relays which repeat the currents corresponding to the depressed keys and actuate electromagnets in the receivers.
Each receiver is provided with five electromagnets corresponding to the five keys of the keyboard, and the armatures of the electromagnets can thus repeat the various combinations for all the signals allocated to the different combinations of the keys.
The instrument was joined in circuit with a battery and another similar instrument placed at a distance; and a continuous current was made to flow through the circuit, keeping the electromagnets energized.
It was originally the practice to place the calling apparatus in series in the line circuit, but the effect of the large impedance introduced by the electromagnets of the call XXVI.
With suitable arrangements of iron and coil and a sufficiently strong current, the intensity of the temporary magnetization may be very high, and electromagnets capable of lifting weights of several tons are in daily use in engineering works (see Electromagnetism).
For the greatest possible " lifting power " of permanent magnets this estimate is probably not very far from the truth, but it is now clearly understood that the force which can be exerted by an electromagnet, or by a pair of electromagnets with= opposite poles in contact, is only limited by the greatest value to which it is practically possible to raise the magnetizing force H.
Joseph Henry, in the United States, first suggested the construction of what were then called intensity electromagnets, by winding upon a horseshoe-shaped piece of soft iron many superimposed windings of copper wire, insulated by covering it with silk or cotton, and then sending through the coils the current from a voltaic battery.
Subsequently similar machines with electromagnets were introduced by Henry Wilde (b.
Wilde introduced the use of electromagnets for the field magnets.
In June 1828 and in March 1829 he exhibited before the institute small electromagnets closely and repeatedly wound with silk-covered wire, which had a far greater lifting power than any then known.