A number of "drops" may be arranged on a board and placed in different circuits so as to indicate which circuit is closed at any time. It is a good plan to arrange a bar to act as a stop, so that the numeral will not drop down too far. Each time that any one of the drops falls, it must be reset by pushing the bar back into position.
CHAPTER X ELECTRIC TELEGRAPHS
Experiments in telegraphy were carried out as far back as the year 1753, when it was proposed to transmit messages by representing the letters of the alphabet by combinations of sparks produced by a static machine; but these were of little practical value and nothing of any importance was accomplished until after the discovery of galvanic current.
Many of these old experiments were very crude and appear somewhat ridiculous when compared with the methods of nowadays. The earliest proposal for an electric telegraph appeared in the Scots’ Magazine for February, 1753, and shows several kinds of proposed telegraphs acting by the attractive power of electricity, conveyed by a series of parallel wires, one wire corresponding to each letter of the alphabet and supported by glass rods at every twenty yards. Words were to be spelled by the action of the electricity in attracting paper letters, or by striking bells corresponding to letters.
The modern telegraph consists essentially of four things, namely:
A battery which produces an electric current.
A wire which conducts the electric current from one point to another.
A transmitter for shutting the current off and on.
An electro-magnetic receiving apparatus, which gives out in sounds, the signals made by the pulsations of the current from a distant point.