galvanometer G, due to the full potential of the charging battery. K is then opened after a suitable charging period has elapsed, and the deflection on the galvano

meter gradually falls owing to the discharging of C through the resistance R. The reduced deflection dl at the end of S seconds from the commencement of the

In both the above tests C is in microfarads, S in seconds, and R in megohms.

Having thus far dealt with the elementary principles for the practical determination of the four fundamental Electromotive electrical quantities, viz., Resistance,

Force, Current, and Capacity, we will now proceed to discuss one or two modifications and combinations of the foregoing methods, together with the apparatus necessary for their conduct, which are applicable to certain branches of electrical work.

Cable testing, in its many and varied forms, probably calls for the largest combination of electrical tests to discover its various qualities before being put on the market, and after being fixed or laid in position, in order to ascertain if it is suitable for the work it is intended to

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perform in the transmission of electrical energy from one point to another.

Electric cables and wires are tested for insulation resistance, electrostatic capacity, ohmic resistance, and, in the case of high tension work, for resistance to disruptive discharge, both at the factory during manufacture and by the purchaser when fixed in position, before absolutely putting them to work, and several very useful combinations of apparatus have been designed, with a view to including two or more of these various tests, and the switches, shunts, resistances, etc., necessary to their manipulation upon one universal base for the sake of portability and general convenience. Of these special combinations we will select two, and deal with them in turn as a pattern upon which are moulded the remainder of their species.

The Silvertown Portable Testing set, a most popular

combination with electrical engineers generally, is depicted in toto in the accompanying block, together with a general diagram of the connections involved in its construction.

It consists in the main of a small dial pattern bridge, and horizontally pivoted galvanometer, together with the necessary shunts and. circuit key, plug switches, etc., necessary to render it adaptable at will to the measurement of both conductor and insulation resistance of all sorts and conditions of electrical circuits. The whole of the necessary detail is mounted on an ebonite base piece, let into the upper portion of a strong, serviceable,

polished wood base, provided with a sunk cover, specially lined and padded to receive the plugs and other loose portions of the apparatus when not in use. By an ingenious attachment, the act of closing the lid raises the galvanometer needle from its pivot, and secures it from injury consequent on rough handling. The makers also supply a second convenient box containing the batteries necessary for the conduct of the aforementioned tests, viz., a set of three low resistance Leclanché elements for the measurement of conductor resistance by the bridge method, where an appreciable current is required in the circuit, and, in addition, 36 small Leclanché elements, capable of yielding a total voltage of 55, on open circuit, for the measurement of insulation resistance by the previously described direct deflection method. No appreciable current can be taken from these cells without materially lowering their voltage, but, in the test under consideration, such a course is not necessary, the main object being the provision of a sufficient electromotive force, or difference of potential between the conductor of the cable under test and earth.

As will be noted from the accompanying illustration and diagram of connections, there are two sets of dial resistances, tens and units respectively, in the variable arm of the bridge, whilst the proportional arms run in thousands, hundreds, and tens respectively. The connections between the battery and testing set consist of flexible leads provided with ebonite capped plugs, which fit into holes to the right of the set, marked INSULn. and BRIDGE respectively, according to the test which it is required to make. There are three shunts, so marked in the diagram, giving multiplying powers of 5, 20, and 100 respectively, according to which of them is introduced. into the circuit by inserting the plug in its corresponding position. The plug blocks marked 50,000 ohms and INSULn. are used when making an insulation test, as will be described presently, whilst the upper pair of terminals are for insulation, and the lower pair for conductor resistance measurement respectively. We will take the two tests of which this compact set is capable in turn, and deal with the mode of procedure in either case. Before proceeding further, however, I may mention that a controlling magnet for the galvanometer is

provided in swivel form, countersunk into the side of the containing case as shown, and its effect is such that, with its north pole uppermost, the galvanometer is most sensitive, and vice versa.

To proceed, however, with the Wheatstone bridge method of resistance measurement by this apparatus. The resistance to be measured is firmly connected to the two lower terminals marked BRIDGE TERMINALS, and the battery leads having been connected to the three low resistance Leclanché elements in the battery set, the plugs are inserted in the holes marked BRIDGE, thereby supplying current to the system. The key on the left