The needle (in the modern pattern) is of soft iron and is kept magnetized inductively by the action of two permanent steel magnets. The coils are wound with copper wire (covered with silk) to a total resistance of 200 ohms. The actual current required to work the instrument is 3-3 milliampères equivalent approximately to the current given by one Daniel cell through 3,300 ohms, but in practice a current of 10 milliampères is allowed.
A single but important addition to enable the reading from the instrument to be effected by sound, in this arrangement the needle strikes against small tubes formed of tin plate and by this means the movement of the needle to the right or left is quite audible.
The Wheatstone automatic apparatus is largely used, especially for press matter; through it a speed of 600 letters per minute can be obtained.
In the Rowland multiple method, the transmitter consists of a mechanical key board, provided with a series of levers which effect certain combinations of positive and negative currents for each letter: these currents are furnished by an alternator, which transmits sine currents over the line, both machines running in synchronism.
At the receiving end of the circuit a shaft is coupled to the motor; this is provided with gearing which rotates four combining commutators and four type wheels, which print the letters on the band of paper. There are four transmitters and four receivers, which are operated independently by means of an adaptation of the multiplex system of working, and each circuit is provided with a number of segments set apart for its own use. Each transmitter is, therefore, able to transmit a separate series of positive and negative currents in different combinations; these are distributed by suitably arranged distributors and relays at the receiving end of the line into their respective receivers. The function of the “combiner” in each receiving instrument is to group the received combinations of positive and negative currents that they operate polarized relays in such a manner that the position of the tongues corresponds with the operation of the levers in the transmitter, since each letter is represented by a specific combination of positive and negative currents.
It is possible by means of the combinations to close a local circuit at any given interval and so cause the paper to be pressed against the periphery of the type wheel at the time when the letter required is opposite.
The paper is also caused to advance automatically for each letter, start a first line, and also to commence a fresh form at the completion of each message.
The Delaney Multiples System, which has been adopted to a limited extent in Great Britain, enables a large amount of work to be done on one wire.
Between London and Manchester four sets of the apparatus can be worked, but between London and Birmingham, a shorter distance, six sets (the maximum for which the system is adopted) may be used.