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

gives a multiplying power of 20, which is equivalent to the deflection produced by the battery current through a standard resistance of one megohm. The true deflection thus obtained, or, in other words, the observed deflection multiplied by 20, gives us the insulation "constant," which term has been already explained. The plug is then withdrawn and re-inserted in the opening marked INSULn., when the deflection due to the cable, with or without a shunt, will be obtained. It remains, therefore, to divide the latter deflection into the former to ascertain the insulation resistance of the cable under test in megohms.

The key on the left may, if all the values in the proportional arms of the bridge be plugged up, be employed as a short circuit key across the terminals of the galvanometer, and, in this capacity, is useful for checking the oscillations of the needle.

By employing the maximum shunt, i.e., the shunt giving a multiplying power of 100, a larger testing voltage than that described above may be employed with the same result, except that the constant will then be through an equivalent of five megohms instead of one, which fact must be taken into consideration in calculating results.

The following additional rules concerning this test are also excerpted from the manufacturers' instructions, and should be rigidly adhered to, if satisfactory results are to be obtained.

"(1) Too much care cannot be taken in preparing the ends of the cable. Since we are measuring a very small current of electricity passing from the conductor to the outside sheathing, through the insulated covering, it is clear that our results will be entirely misleading if any current be allowed to pass over a dirty surface at the ends where the conductor is exposed. These ends should be looked to before testing, and in the case of india-rubber or other firm material, the section of the insulator should be pared all over with a sharp and perfectly clean knife.

'(2) Care should be taken not to short-circuit the battery, which may easily occur in two ways. One is by allowing the two battery plugs to touch one another, when the other ends of the leads are attached to the

battery terminals; and another is by allowing the lead attached to the earth terminal to touch that attached to the insulation terminal.

"In both cases the battery of small cells will be for a time much overworked, and in the second the needle may become bent or demagnetised.

(3) Another point that may be noticed is that in deducing the insulation resistance per statute mile from a test on any given length, the result obtained from a test on the latter is to be multiplied by the length of the piece in miles, and not divided by it."

Such a combination of instruments and apparatus as the one just described is admirably suited for installation work such as falls under the duties of mains inspector, wiring contractor, etc., etc., and, in fact, in all cases where a convenient portable testing set is required and

Norman Testing Switch for Insulation and Capacity Tests on Electric Cables and Circuits.

extreme accuracy is not essential, but, as its range is somewhat limited, it is unsuited for making tests on cables at the factory or in cases where comparatively high insulation resistances, often amounting to some thousands of megohms, have to be dealt with.

A switch and shunt combination which has been designed to meet the requirements of such cases, as also with a view to portability and general convenience, is that known as the Norman Testing Switch, designed by Mr. H. D. Norman, which by successive movements of a single lever serves to make and break the various circuits necessary in testing a cable or circuit for electrostatic capacity, insulation resistance, and consequent earth readings, together with the introduction of the necessary shunts, all of which are included upon the same base.

A general view of the switch is shown in the accompanying block, to which is appended a general plan dia

E

S

AO

B

FIG. 68.

gram of connections, Fig. 68, showing the mode of connecting extraneous apparatus to the switch. Referring

Earth