File:EB1911 Telegraph - Single Needle with sounding arrangement.jpg

English: At telegraph offices where the reception work is heavier than can be dealt with by the A B C apparatus, the “Single Needle” instrument has been very largely employed; it has the advantage of slight liability to derangement, and of requiring very little adjustment. A fairly skilled operator can signal with it at the rate of 20 words per minute. 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), 10 mils. in diameter, to a total resistance of 200 ohms. The actual current required to work the instrument is 3.3 milliamperes (equivalent approximately to the current given by 1 Daniell cell through 3300 ohms), but in practice a current of 10 milliamperes is allowed. A simple, but important, addition to enable the reading from the instrument to be effected by sound is shown; in this arrangement the needle strikes against small tubes formed of tin-plate.