The inventor of the electromagnetic recording telegraph and of the dot-and-dash alphabet known by his name, was born at Charlestown, Massachusetts, graduated at Yale College in 1810, invented the commercial telegraph in 1832, and struggled for twelve years in great poverty to perfect it and secure its proper presentation to the public. The first public exhibition of the completed instrument was made in 1837 at New York University, signals being sent through 1700 feet of copper wire. It was with the aid of a $30,000 grant from Congress that the first commercial line was constructed in 1844 between Washington and Baltimore

(Fig. 269, 4), which is so arranged that it clicks both when it is drawn down by its electromagnet against the stop S and when it is pushed up again by its spring, on breaking the current, against the stop t. The interval which elapses between these two clicks indicates to the operator whether a dot or a dash is sent. The small current in the main line simply serves to close and open the circuit in the local battery which operates the sounder (see drawings on opposite page). The electromagnets of the relay and the sounder differ in that the latter consists of a few hundred turns of coarse wire and carries a comparatively large current.

315. Plan of a telegraphic system. The actual arrangement of the various parts of a telegraphic system is shown in the drawings on the opposite page. When an operator at Chicago wishes to send a message to New York, he first opens the switch which is connected to his key, and which is always kept closed except when he is sending a message. He then begins to operate his key, thus controlling the clicks of both his own sounder and that at New York. When the Chicago switch is closed and the one at New York open, the New York operator is able to send a message back over the same line. In practice a message is not usually sent as far as from Chicago to New York over a single line, save in the case of transoceanic cables. Instead it is automatically transferred, say at Cleveland, to a second line, which carries it on to Buffalo, where it is again transferred to a third line, which carries it on to New York. The transfer is made in precisely the same way as the transfer from the main circuit to the sounder circuit. If, for example, the sounder circuit at Cleveland is lengthened so as to extend to Buffalo, and if the sounder itself is replaced by a relay (called in this case a repeater), and the local battery by a line battery, then the sounder circuit has been transformed into a repeater circuit, and all the conditions are met for an automatic transfer of the message at Cleveland.

QUESTIONS AND PROBLEMS

1. Draw a diagram showing how an electric bell works.

2. Draw a diagram of a short two-station telegraph line which has only one instrument at each station.

3. Draw a diagram showing how the relay and sounder operate in a telegraphic circuit. Why is a relay used?

RESISTANCE AND ELECTROMOTIVE FORCE

316. Electrical resistance.* Let the circuit of a galvanic cell be connected through a lecture-table ammeter, or any low-resistance galvanometer, and, for example, 20 feet of No. 30 copper wire, and let the deflection of the needle be noted. Then let the copper wire be replaced by an equal length of No. 30 German-silver wire. The deflection will be found to be a very small fraction of what it was at first.

A cell, therefore, which is capable of developing a certain fixed electrical pressure is able to force very much more current through a given wire of copper than through an exactly similar wire of German silver. We say, therefore, that German silver offers a higher resistance to the passage of electricity than does copper. Similarly, every particular substance has its own characteristic power of transmitting electrical currents. Since silver is the best conductor known, resistances of different substances are commonly referred to it as a standard, and the ratio between the resistance of a given wire of any substance and the resistance of an exactly similar silver wire is called the specific resistance of that substance. The specific resistances of some of the commoner metals in terms of silver are given below:

The resistance of any conductor is directly proportional to its length and inversely proportional to the area of its cross section or to the square of its diameter.

The unit of resistance is the ohm, named after Georg Ohm (see opposite p. 268). A length of 9.35 feet of No. 30 copper

*This subject should be accompanied and followed by laboratory experiments on Ohm's law, on the comparison of wire resistances, and on the measurement of internal resistances. See, for example, Experiments 32, 33, and 34 of the authors' Manual.

wire, or 6.2 inches of No. 30 German-silver wire, has a resistance of about one ohm. The legal definition of the ohm is a resistance equal to that of a column of mercury 106.3 centimeters long and 1 square millimeter in cross section, at 0°C.

317. Resistance and temperature. Let the circuit of a galvanic cell be closed through a galvanometer of very low resistance and about 10 feet of No. 30 iron wire wrapped about a strip of asbestos. Let the deflection of the galvanometer be observed as the wire is heated in a Bunsen flame. As the temperature rises higher and higher the current will be found to fall continually.

The experiment shows that the resistance of iron increases with rising temperature. This is a general law which holds for all metals. In the case of liquid conductors, on the other hand, the resistance usually decreases with increasing temperature. Carbon and a few other solids show a similar behavior, the filament in the early form of incandescent electric lamp having only about half the resistance when hot which it has when cold.

318. Electromotive force and its measurement.* The potential difference which a galvanic cell or any other generator of electricity is able to maintain between its terminals when these terminals are not connected by a wire - that is, the total electrical pressure which the generator is capable of exerting is commonly called its electromotive force, usually abbreviated to E.M.F. The E.M.F. of an electrical generator may be defined as its capacity for producing electrical pressure, or P.D. This P.D. might be measured, as in § 294, by the deflection produced in an electroscope when one terminal is connected to the case of the electroscope and the other terminal to the knob. Potential differences are, in fact, measured in this way in all so-called electrostatic voltmeters.

*This subject should be preceded or accompanied by laboratory work on E.M.F. See, for example, Experiment 31 of the authors' Manual.