Saturday, September 11, 2021



The use of electricity for purposes of communication is so widespread that site would be incomplete without some reference to the principles involved in simple telephone and telegraph circuits.


Sound consists of vibrations, usually in the air, which may be compared to ripples on the surface of a pond. Instead of being confined to a surface, however, the air vibrations spread out in all directions from their source. Moreover, while the horizontal movement of a ripple across a pond is produced by up-and-down movements of the particles of water, the vibration of the particles of air takes place in the same direction as that in which the sound is transmitted.

The frequency of the vibrations settles the pitch of the sound, a low frequency producing a low note and a high frequency a high note. The frequency of the notes on a piano ranges from thirty to three thousand cycles per second (30 to 3000 Hz). The ear can detect frequencies of from about 20 to 20,000 Hz.

The sound-waves have a waveform which can be represented by a sine curve (SEE ARTICLE) in simple cases, but which is very complex in the case of speech. The problem of telephony is to reproduce the sound waves at a distant point. This is done by the use of a transmitter or microphone which converts the sound-waves into corresponding variations in the strength of an electric current and a receiver that responds to the varying current and generates new sound-waves similar to those which affected the transmitter.


The transmitter commonly employed is a special type of variable resistance controlled by the sound-waves. An elementary form is shown below.


A flexible conducting diaphragm is separated from a rigid conducting disc by a number of small granules of carbon g. External connections are made to the diaphragm and the rigid disc r. Current passing through the transmitter has to traverse a large number of more or less imperfect contacts between the carbon granules, and the resistance of this oath is very sensitive to variations in pressure on the diaphragm d.

The diaphragm vibrates in response to any sound-waves directed towards it, thus producing varying pressure on the granules. The resistance of the transmitter is therefore continually altering in accordance with the frequency and waveform of the sound waves.

The receiver consists of a magnet m, a coil, and an iron diaphragm. The coil is in series with the transmitter and a cell. As the current which the cell is able to send round the circuit is varied by the transmitter, the coil at the receiver produces corresponding variations in the flux of the magnet. The result is to vibrate the iron diaphragm, thus generating new sound waves similar to those directed towards the transmitter.

It is of interest to note that the receiver itself will together be used as a telephone. When soundwaves are directed onto the diaphragm, its movement results in a continual redistribution of the flux produced by the magnet. The changing flux cuts the coil and induces currents in the winding corresponding to the speech waves. These currents affect the second receiver in the usual manner. Provided that the instruments are fitted with permanent magnets to produce the initial flux, no battery is required.


Many modern commercial transmitters of the variable-resistance type employ two diaphragms, an outer one for receiving the sound waves and a small inner one forming part of the granule chamber. The two are joined at their centers. The carbon granules are situated between two carbon discs, one attached to the back of the inner diaphragm and the other to the back of the granule chamber. All the contact points influenced by the sound waves are thus between one carbon surface and another.

For special purposes, transmitters based upon other principles are employed. By way of example, we may mention moving-coil microphones, in which a coil coupled to the diaphragm moves in a magnetic field, and thus has speech currents induced in it, and electrostatic microphones in which the diaphragm forms one plate of a capacitor the capacitance of which varies in accordance with the speech waves.

Commercial receivers do not differ in principle from the simple form is shown, but the magnetic circuit is improved by the use of a magnet of horseshoe or some equivalent form. Both the pole-pieces can thus be brought close to the diaphragm. A separate coil is usually fitted to each pole-piece, the two coils being connected in series.


A complete circuit between two telephones must make provision for a transmitter and receiver at each end. One possible arrangement is to connect all four instruments in series and to insert a battery at any convenient point in the circuit. If, however, the line is a long one, the resistance of the transmitters will then form only a small part of the whole, and the variations in current caused by the speech waves will be slight.

This difficulty can be overcome by connecting each transmitter in a local circuit in series with its own source of current and the primary winding of an induction coil. The latter is merely a small straight-core transformer of the kind shown in the previous article. The secondary winding of the induction coil is included in series with the line as shown in the diagram above.

The variations in the transmitter resistance produce large current variations in the local circuit, and these, in turn, produce large voltage variations across the secondary winding of the induction coil. The result is a much more effective transmission circuit than would be possible if the transmitters were connected directly in series with the line.


 In a public telephone system, each telephone is normally provided with its own pair of wires to the telephone exchange, at which arrangements are made to connect any pair of wires to any other pair. In modern exchange systems, local batteries at the telephones are not used, all the transmitters being supplied with current from a common battery at the exchange. Special circuit arrangements are then necessary to retain the advantages of the local transmitter circuit to ensure that the varying speech currents between one pair of telephones connected to the common battery do not reach other telephones drawing their current from the same source. For particulars of these circuits, the reader is referred to specialized books on telephony.

Calling is nearly always affected by means of alternating current connected to the line at the exchange. In order that it may respond properly to this current, the bell at the telephone instrument is fitted with a permanent magnet, which causes the armature to move in one direction in response to one-half cycle of current and in the opposite direction in response to the next half-cycle. Bells of this kind are said to be polarized.


One of the earliest applications of electricity was to the telegraph, and this is still an important means of communication. In its simplest form, an electric telegraph consists simply of a source of current, a key for opening and closing the circuit at one end of a line, and a device responsive to the current at the other. Signaling can be carried on over such a circuit by means of the morse or any other code.

The signal receiving device may be a modified galvanometer, a sounder consisting of an electromagnet the armature of which is designed to give a distinctive click at each operation, or a relay arranged to close a local circuit for some other indicating or recording instrument when the line current flows.


If the circuit is simply opened and closed at the sending end, it is said to be arranged for single-current working. This is satisfactory for short lines on which the capacitance is small, but as the capacitance increases, rapid signaling is prevented by the time taken to charge the line when the key contacts are closed and to discharge it when they are opened.

An improvement can be effected by filling up the gaps between one impulse and the next by current flowing in the opposite direction. This is known as double-current working and necessitates the use of a receiving device the response of which is dependent upon the direction in which the current flows. The current constituting the signals proper is called the marking current, and that flowing in the opposite direction is the spacing current. Contacts on the transmitting key are arranged to send a spacing current when the key is at rest and a marking current when it is depressed.


A simplex telegraph circuit is one in which a message can be sent in one direction at a time. It is possible to send non-interfering messages in both directions at the same time by means of what is called a duplex circuit. The diagram below shows the principle of one form.

The signals are received by relays, each of which has two windings, as represented conventionally at the diagram r r’.

In addition to its relay, each station has a key K K’, a cell C C’, and a line balancing resistance b b’. The object of the resistance will appear very shortly.

With the keys in the position shown, no current flows. When the key k is pressed,  the cell C ends a current through the upper windings of both relays, the circuit being completed through the back contact of the key  K’ and the earth return. No current flows in the lower windings of relay r’, as this is short-circuited by the key, but the relay is operated by the current in its upper winding.

This current also flows through the upper winding of the relay r, but in this case there is another circuit through the lower winding and the line balancing resistance b. The latter is chosen so as to make the current equal to that in the line circuit, and as the two windings oppose each other, the relay does not operate.

Suppose that while key R  is still depressed, key R’ is depressed also. Cells c and c’ are now in opposition, and there is, therefore, no current in the line. Current flows, however, in the lower windings of both relays, so that relay r operates while relay r’ remains in its operated position.

If key k is now released, current from cell c’ flows over the line. Relay r is therefore held in its operated position by current in its upper winding, while relay r’, which now has current flowing in both windings, releases.

The result is that each relay responds to the movements of the key at the other end of the line and ignores the movements of its own. When either relay operates, the armature closes a circuit for any desired form of indicating device.

Direct-current working has now been largely abandoned in telegraph systems, but the duplex circuit is still worthy of notice as an example of the unexpected results obtainable by the application of ingenuity to basic principles.


The development of the telephone has greatly influenced the technique of telegraphy, and many modern telegraph systems make use of voice-frequency currents, i.e., alternating currents the frequency of which is within the range to which the ear responds. We mentioned in previous articles that combinations of inductance and capacitance possessed a resonant frequency at which a maximum current would flow. By an extension of this principle, it is possible to design circuits that will accept currents of one frequency while rejecting those of another. Several messages can therefore be sent over the same line by using a different frequency for each, the various frequencies being sorted out at the receiving end and passed to separate receiving apparatus.


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