Another very useful form of reversing key is Pohl's mercury commutator (see Fig. 7). This key may not only be used as a reversing key, but also for connecting either of two circuits to a third, which is very useful, for instance, in comparing the fall of potential in an unknown resistance.

with that in a standard resistance. The following diagrams show the connections in the two cases :

(1) As an ordinary reversing key (Fig. 8). In one position of the rocker, I is connected to 3 and 2 to 4; in the other posi

FIG. 7.

tion, I is connected to 5 and 2 to 6; contacts 3 and 6, also 4 and 5, being permanently connected by copper straps.

(2) For connecting either of two circuits to a third, the two copper straps are removed (Fig. 9).

When I is connected to 3 and 2 to 4, the first and third circuits are connected together; also where I is connected to 5 and 2 to 6, the second and third circuits are connected.

This

18. High-Insulation Keys.-In cable and condenser work, keys of special construction and of high insulation are required. One form largely used is shown in Fig. 10. consists of a movable brass tongue attached to a long ebonite pillar. The tongue moves between an upper and lower contact, both of which are highly insulated from the base; there

are also two detents attached to levers marked "discharge and "insulate." When the tongue is depressed, it is held on the lower contact by the detent attached to the "insulate" lever. If this lever is depressed, the tongue springs up until it is caught by the other detent, which holds it midway between the two contacts. When the lever marked "discharge" is depressed, the tongue springs up to the upper contact. The ebonite insulating pillars should be corrugated, in order to offer a greater resistance to surface leakage; and the screws by which they are attached to the base should only enter the ebonite a short distance, otherwise the advantage of all the insulation is lost. Such keys, and in fact all keys,

should be carefully cleaned before being used. The ebonite should occasionally be well washed with soap and distilled water and dried before a good fire; drying by rubbing is apt to electrify the ebonite, and this in some cases would introduce errors into the experiments. Such instruments also should always be lifted by the metal part, as the hand is apt to leave a film of moisture on the ebonite which might spoil its insulating properties.

19. Batteries.-In many experiments one of the necessary requirements is a battery of constant E.M.F. No ordinary primary battery will fulfil this condition, unless the current taken from it is very small and is only required for a short interval of time. When large currents are required, or a constant E.M.F., for any length of time, a secondary battery

must be employed. Small-size secondary batteries of about 14 ampère hours' capacity will be found very useful for general laboratory work, and they may be relied upon to maintain a constant potential difference at their terminals, provided they are not used immediately after being charged, but after having been partially discharged. Great care must be taken not to accidentally short circuit such cells, since the internal resistance is very small, and a large current might not only damage the cell but also the external resistance. It will sometimes be found a good plan to make up sets of three such cells in boxes, inside which is a resistance in series with them, sufficient to protect the cells from a dangerous short circuit.

Dry batteries and Leclanchés will be found very useful for ordinary testing and bridge work, where the constancy of the current is not of such vital importance. The latter cells will be much improved if potassium permanganate crystals are mixed up with the manganese doxide in the porous pot; and if this cannot be done, the cells may be "revived" from time to time by pouring a solution of potassium permanganate into the porous pot through one of the air tubes. Other forms of battery, such as Daniell's and Latimer Clark's cells, will be dealt with later on in connection with standard cells.

20. In many cases an E.M.F. very much smaller than that given by any of the above batteries, but of known value, is

A

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B

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FIG. II.

required; this may be obtained as follows. A current from an ordinary cell is sent through a very high resistance, say 10,000 ohms, in series with a low-resistance sensitive galvanometer (see AB in Fig. 11). The terminals A, C supply a potential. difference which may be any desired fraction of AB, by simply

moving C nearer to or farther from B. The object of the galvanometer is to enable the potential between A and C to be kept constant when the resistance across AC varies; as the external circuit attached to AC diminishes in resistance, the potential difference between A and C will fall, and this will be indicated by the galvanometer deflection falling off. The point B must now be adjusted so as to diminish the total resistance between A and B until the galvanometer deflection is restored to its original value, the P.D. between A and C then being the same as before, and the same fraction of the original value of AB, the fall of potential down which may be taken as equal to the E.M.F. of the cell.

21. Resistances.— In addition to the ordinary resistance boxes, post-office bridges, etc., which are accurately adjusted, but meant only to carry small currents (in no case more than o'001 ampère should be allowed to pass through them), resistance coils are often required, which, while still being accurate, are capable of

FIG. 12

carrying much stronger currents without heating to such an extent as to alter their resistance appreciably. Such coils may be made of manganin, wound in non-inductive spirals and stretched on a large wooden frame, so as to expose a large cooling surface, the ends of the coils being soldered to thick copper wires; a very convenient form used by the author being that

shown in Fig. 12, one of the copper end pieces being shown in Fig. 13, and consisting of a piece of thick copper wire first doubled in two, the end of the resistance coil being soldered into the bend, and then one

The following table gives the sizes of wires that may be employed for the various resistance coils :

The wires in all cases are double silk covered. Also, in cutting the lengths of wire for the coils, it is advisable to test each gauge for specific resistance, as various specimens of manganin are found to vary among themselves. The coil, before being finally adjusted, should be artificially aged by heating to a temperature of 140° C. in an air-bath for 5 hours; some authorities also recommend coating the wire with shellac varnish, to protect it from the oxidizing action of the air. The advantage of using manganin is its high specific resistance and negligible temperature coefficient.

22. In addition to such resistances as the above, variable unknown resistances are often required to regulate the current in a circuit. For purposes of this kind, Varley's carbon rheostat (see

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