d. Current adjust potentiometer. You use this potentiometer and a milliammeter (not shown) to adjust the
current by changing the resistance in the teletypewriter circuit.
FIGURE 79. Principal Parts of a
The One-Way-Only Teletypewriter Circuit.
a. The simplest of teletypewriter circuits is used to transmit messages in one direction only. Hence, it is called a
one-way-only circuit (fig. 80). In this circuit the receiving teletypewriter is simply a monitor. A common application is
the ticker-tape machine in the office of a stock broker.
b. The four principal parts of a teletypewriter are connected in series in the sending teletypewriter. In the
receiving teletypewriter there is no keyboard and transmitting contacts are unnecessary. The sending and receiving
teletypewriters are connected by a wire or cable known as the line.
c. In a ground-return circuit such as that shown in figure 80, each teletypewriter is grounded by means of a
grounding rod. The earth thus provides a return path for current flow. Most of the circuits in this text are shown with a
ground-return path to make the circuit drawings easy to understand. In practice, however, full-metallic circuits are usually
used. For a full-metallic circuit, all the ground points at the sending station are connected to all the ground points at the
receiving station by an additional line wire or cable. Full-metallic circuits offer less resistance to current flow. Also, a
full-metallic circuit must be used for classified message traffic. It makes it more difficult for the enemy to intercept the