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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 conpections 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 air 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
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 versâ.
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
completes the galvanometer circuit, and we proceed to obtain a balance in the usual manner. The following simple rules issued by the manufacturers for the preliminary setting of the proportional arms where the value of the resistance under test is approximately known, will be found useful :
“For resistances between 1 ohm and 10 ohms, lefthand coil 100 ohms; right-hand coil 10 ohms.
“For resistances between 10 ohms and 100 ohms, lefthand coil 100 ohms, right-hand coil 100 ohms.
“For resistances between 100 ohms and 1,000 ohms, left-hand coil 100 ohms, right-hand coil 1,000 ohms.”
The third figure can in all cases be obtained by interpellation, in the usual manner, employing the formula given under the heading of Resistance Measurement by Wheatstone Bridge Method, previously described in these paragraphs.
Infinity is obtained by withdrawing one or both of the dial plugs.
The following additional rules, issued by the manufacturers in respect of this test, should also be duly noted in using the apparatus :
Except in testing at the extreme range of the instrument, i.e., quantities less than one ohm or greater than 1,000 ohms, the galvanometer will be found amply sensitive, and it is better to place the south end of the controlling magnet uppermost, thereby reducing the time of the oscillations of the galvanometer needle.
“The battery should be in circuit as short a time as possible to avoid running down the cells, and it is well to take out one of the battery lead plugs when any
alterations are being made in the plug commutators, only replacing it just before pressing the galvanometer key.
Care should be taken to connect the conductor to be tested very securely to the bridge terminals. This may be done for very large or stranded conductors, either by soldering to their ends thin brass plates with holes in them of a suitable size to go under the heads of the terminals, or the connection may be made by means of finer wires soldered to the end of the main conductor. The resistance of these must be independently ascertained, and subtracted from the gross result.”
The insulation test is conducted in a slightly different
manner, and may be described as follows:- The insulation testing battery of 36 to 39 cells is connected by the flexible leads and concomitant plugs with the two holes marked INSULn., the cable under test and its earth or sheathing having been previously connected to the two upper terminals marked INSULn. and EARTH respectively. Referring now to the left-hand top plug switch, the plug is first inserted in the opening marked 50,000 OHMS in order to take the constant. This, with the available voltage of the accompanying set of cells, will be found to require the insertion of that shunt which