TILE TRANSMITTING KEY. The 'key' by which signals are transmitted from the operator at the sending office is shown below in its usual form. It consists of a lever pivoted near the mid dle and carrying at one end a knob by which it is manipulated by the operator in sending signals. Two platinum contacts are fastened respectively to the under side of the lever and to an insulated piece on the upper side of the base. These con tacts are exactly opposite to each other, so that they come every time the key is de pressed. The lever is held up, when the key is not depressed by the operator, by a small spiral spring. One end of the line wire is connected to the sounder and then to the metallic base of the key from which the current passes through the pivots to the lever and the small contact point attached to its under side. One terminal of the battery is connected with the small insulated piece supporting the lower contact while the other goes to the ground or return wire. By this arrangement no current can pass through the line until the key or lever is depressed, since the terminal of the wire which is fastened to the contact is insulated and the current has no means of reaching the other part of the wire. When it is desired to send the current the knob is depressed, and the current from the battery, finding a passage between the two contacts, passes from one end of the wire to the other and proceeds upon the line wire to the distant station. The length of time during which the key is de prosscd determines the length of time between the upward and downward clicks of the sounder at the other end of the line. Thus if the operator
presses down the handle and releases it quickly the keeper of the sounder will go down and up, immediately indicating a 'dot' or the letter `E,' while if he presses the key and holds it down a moment before releasing it, there will be a longer interval between the downward and upper clicks of the sounder, and a 'dash' or the letter 'T' will be produced. In early instruments, before the operators read the messages by the clicks, the receiving instrument was arranged to make a record of the signals upon a moving strip of paper. For this purpose a pen or sharp point was attached to a lever, and a strip of paper, kept in motion by clockwork, was arranged to pass un der the pen, so that a mark was produced by the pen every time the lever was pulled down by the magnet. If the lever was immediately released a dot was recorded, but if the lever was held down a moment a longer mark called a dash was produced.
The signals used at first were arrangod for making a record with a crude instrument of this kind, and while the increasing skill of the operators soon enabled them to read the signals directly from the clicking of the instrument with out looking at the tape, the signals are still spoken of as consisting of so many dots and dashes, as if recorded on paper. These registers are to-day but little used, being seen occasionally in district telegraph stations or with fire-alarm apparatus.