This diagram shows a Morse telegraph circuit connected between two distant points. The point at which telegraph messages are sent and received is called an "office". In this case, each office is equipped with three instruments and two batteries. The
purpose of each instrument is explained below.

K = Telegraph key.  This is the basic "transmitter". A telegraph key is nothing more than a simple electrical switch. This switch has a pair of contacts that makes and breaks the circuit. The key is ergonomically designed to allow the operator to rapidly make and break the contacts in order to transmit symbols of the Morse code.

R = Morse Relay.  The Morse relay is a simple electromechanical amplifier. The Morse relay is designed to be extremely sensitive to electric current. A horseshoe electromagnet is arranged to attract a lightly balanced iron armature. A contact on the armature and on a stationary post make contact when current flows through the coils of the electromagnet. These contacts make and break the "local circuit".  When adjusted very carefully, it is possible for a current of less than 1 milliampere to operature a Morse relay, although normally there is more current available and that makes operation more reliable.

S = Local Sounder.  The "sounder" is the receiving instrument. This instrument is designed to produce an audible "click" when its electromagnet is energized. It also produces a second "click" with a slightly different tone upon cessation of the current. Thus each current pulse produces a "click-clack" sound. The local sounder is designed for maximum loudness, and it is not very sensitive to electrical current. It takes as much as 50 or more times the current to activate a local sounder compared to a relay.

MLB = Main Line Battery.  This is a battery made of several electrochemical cells connected in series. Up to several hundred volts may be required depending on the length of the line and the number of instruments cut in to the wire. These batteries were replaced with dynamos somewhere around the turn of the century.

LB = Local Battery.  This is usually a single cell with a voltage of 1 to 1.5 volts. Its only purpose is as a current source for the local sounder. The local battery continued in use well into the 20th century in the many depots and way stations without AC power.

MAIN LINE = Telegraph Line.  Usually a very heavy iron wire is used. The wire is galvanized to reduce corrosion. The telegraph line is suspended from wooden poles using specially designed glass insulators. The design of the insulators is critical in reducing leakage paths to ground. These leakage paths are known as "escapes", and they can render a telegraph line useless in wet weather.

E = Earth.  Connection to earth or ground, through which the main line current returns after passing through the relays and keys, to complete the circuit.  The main resistance here is the actual ground post contact with the earth; the earth itself may present less than 100 ohms resistance over a one hundred mile path, depending on soil conductivity.

Additional offices can be connected in series with the main line (no earth connection required) and may or may not include additional battery voltage for the line.  Note that each office must keep its key CLOSED with a special lever called a circuit closer when messages are not being sent, otherwise the circuit is dead and no one else can send messages!  Every telegraph office has a name or call usually consisting of two letters; thus for New York the call might be NY and for Chicago, CH.  When one station desires to call another the operator opens his circuit closer and repeats the call letters of that station and signs his own call until answered.