poured a layer of oil to prevent the salts, which form, creeping up to the edge of the jar. In setting up the cell, only half of the sticks of caustic potash furnished by the makers should at first be placed in the jar. After water has been poured on them to within an inch of the top of the jar, and they have been dissolved by stirring, the remainder of the sticks may be added and the solution stirred as before. In wiring up the cell, the binding post s connected to the copper-oxide plate forms one terminal, and the binding posts z and z fastened to the zinc plates are coupled together and form the other terminal of the cell. The internal resistance is but 0.025 ohm, and the working electromotive force about 0.75 volt.

Conversing in Both Directions.-Fig. 19 shows a simple telephone circuit over which conversation can be carried on in both directions. It is simply a further development of the principles illustrated in Fig. 8, the apparatus and connections at both ends of the line being identical. As to the receivers it may be questioned why the permanent magnets in them (see Fig. 3) are necessary, now that there is an electromagnetic action to produce the attraction of the diaphragms. The reason is that the permanent magnets are essential for the reproduction of the proper pitch.

A Grounded Telephone Circuit is shown in Fig. 19; that is, the earth or ground is used as one conductor between the two stations a and b, in place of one of the line wires. Either a grounded circuit.

or a two-wire (complete metallic) circuit is practicable in connecting the stations a and b. In the former case, connection with the ground at each. station may be made either by fastening the ground wire to a water pipe or to a metal rod driven down to moist earth. The surface of the pipe or rod should be filed or scraped bright, the wire then wound around it, and a clamp placed over the wire and firmly bolted to the pipe. Another method consists of burying in moist earth below the frost

FIG. 19.-A Grounded Telephone Circuit for Transmitting in Both Directions

line, that is, 5 or 6 feet beneath the surface, a copper plate about 1 foot square to which the ground wire has been riveted, and soldered throughout its length over the plate. The plate should be surrounded on each side with a thin layer of crushed coke or charcoal about pea size to protect it against corrosion from too direct contact with the earth.

Grounded telephone circuits are not usually as satisfactory as complete metallic circuits, especially in the vicinity of electric-light and railway

lines, on account of the inductive disturbances caused by stray earth currents. With good earth connections the grounded portion of the line has a negligible resistance, but at best grounded telephone circuits are serviceable only for lines of moderate length and as the saving in line wire in such cases is small they even then hardly compensate for the risks of disturbances incurred.

The Signal Receiving and Sending Apparatus. In addition to the talking apparatus already considered there must be at each point where a telephone equipment is located, some means of signaling, so that a person A may know when a distant party B wishes to converse with him over the wire, and also to enable A to call B to the telephone. For the purposes here considered, the signal receiver is a magneto bell, and the signal sender is a magneto generator.

The Magneto Bell and diagrammatically in Fig. 20.

Generator are shown
The bell A comprises

an electromagnet cc; also a soft-iron armature v pivoted at its center so as to permit of its oscillating, and, by means of the clapper o attached to it, striking alternately the gongs h and l. The bell is actuated by an alternating current, and depends for its operation on the permanent magnet ns which induces in the armature v a south pole at S and two north poles at the ends N and N. The permanent magnet n s also induces in the yoke u of the electromagnet a north pole at N' and two south poles at the ends s' and s'. As the induced

polarities are approximately equal in strength, and in each case a north pole is opposite a south pole, their combined effect upon the armature v is neutral, and it remains balanced. As soon as an alternating current passes through the magnet coils, which are wound in opposite directions around the cores, this current when flowing in one direction will strengthen one pole of the electromagnet and weaken or reverse its other pole, un

balancing the armature and causing it to swing about its pivot so that the clapper o strikes one of the gongs. At the next instant the alternating current reverses its direction of flow, strengthening the pole which formerly was weakened, and weakening or reversing the other pole of the electromagnet. This will cause the armature to be drawn in the opposite direction and-the clapper to strike the other gong. So long as the alternating current continues to flow, this action will be repeated

and the bell will continue to ring. Direct current, however, would have no effect upon the bell unless an interrupter were used. The action of the permanent magnet in the operation of the armature has led to the instrument being sometimes called. a “polarized” bell or ringer.

The magneto generator B, Fig. 20, is a simple form of alternating-current generator. A coil of insulated wire u, wound upon an iron core e, is revolved by hand in the magnetic field produced by the permanent horseshoe magnet pp. The cutting of the lines of magnetic force by the coil of wire induces in the coil an alternating current which acts upon the magneto bell A as just described. Ordinarily, three or four horseshoe magnets are employed so there will be a strong field; these are arranged side by side with like poles adjacent. Conductors sliding on a pair of metallic rings collect the current generated in the armature. Owing to a high reduction gear being used between the armature shaft and the hand wheel, but a few rapid turns of the latter are necessary to drive the armature at a sufficiently high speed to generate the required power to ring the bell.

In comparison with the ordinary direct-current electric bell and battery, the generator of the magneto set gives a much higher voltage than is usual in the battery, with no trouble from acids and with less cost of operation; the generator and bell being always on closed circuit, there are also fewer contacts to burn, oxidize, or in other ways cause