in the addition of a "shunt," or derived circuit, to each half of the galvanometer: these shunts are marked on the instrument "shunt" they are short wires having a resistance equal to exactlyth of that of the half coil, and when thrown into the circuit by the insertion of the plug, ths of the current pass through them, and only To through the coils of the galvanometer: either one or both half coils may be shunted at pleasure. The instrument is supplied with a set of resistance coils varying from one to ten thousand ohms.

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The instrument does not pretend to minute accuracy, but is well suited for making all the ordinary practical measurements required in telegraphy. Its daily and hourly use cannot be too strongly recommended; for a constant reference to actual measurement is the very foundation of telegraphy. The resistance of every principal circuit should be measured every morning, and its variations recorded. In England it is generally considered that even in bad weather a line should not give less than one megohm (one million ohms) per mile, so that a line of two hundred miles should give not less than = 5000 ohms. If it gives less than this, the low resistance is due to defective insulation. The line should now be tested in many separate sections, either from the office (see Chapter I.) or by a visit to each section. If the resistance per mile is the same for each section the fault is probably in the nature of the insulation, but if, as is usual, some sections are very much

1,000,000

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worse than others the fault will probably be found in tunnels, in contact from branches of trees, from wires or insulators touching the posts, from broken insulators, &c., and a visit to the faulty locality will at once disclose the cause of the evil.

The metallic resistance of the line wire should also be occasionally tested in sections in the finest weather if there is a uniformity in the sections all is well, but if some give an unduly high resistance per mile, a closer examination will almost certainly disclose imperfectlysoldered joints, which oppose a great resistance to the current and interfere seriously with the working; or the earth connections may be out of order. It is difficult for those who have not tried it to believe the improvement that may be made in any line in a few days by quantitative measurements, and by an inspection of the sections which give indications of being defective.

In the same way each battery should be tested for resistance every few days, and those which are faulty should be attended to. The operation is performed in a few moments, and it will frequently be found that one dry or defective cell or battery is sufficient to mar the working of a whole line.

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CHAPTER VIII.

To use the Instrument as an ordinary Galvanometer. PLUG up B and C and attach the line wire and earth to the terminals A and D. The current then passes through half the coil from A to B, thence along the front bar to C, and through the other half coil from C to D. The instrument is now in the condition of an ordinary "detector," and is suited for receiving a current, proving the continuity of a circuit, or detecting a broken connection or loss of insulation in any apparatus. For these purposes the instrument may sometimes with advantage be inclined on its back support.

CHAPTER IX.

To use the Instrument as a Sending and Receiving Instrument.

PUT the line to the terminal marked "key," at the back of the instrument; the battery to one of the small screws on the front bar (the other pole being to earth); connect the terminal marked "bridge" by a wire to A, and put D to earth; lastly, connect a short wire from B to C by means of the flat-headed screws. In this condition, when the key is pressed down, the battery current goes direct from the front bar through the key to line without passing through the instrument. When the key is resting up against the bridge in its normal position, the current received from the line passes from the key through the bridge to its terminal, and thence by the connecting wire to A, and through the galvanometer to D and earth.

If it be preferred that both the transmitted and received current should pass through the galvanometer, the connections are a little varied: join the battery as before to the front terminal and connect a short wire from B to Cas

before; connect a wire from the "key" terminal to A, put the line to D and put "bridge" to earth. In this condition both currents pass through the galvanometer. On emergency the instrument may be used as a receiving instrument by inclining it on its support, and by extemporising stops out of pieces of wood or wire to limit the movement of the needle.