S

The deflections d and dl are, of course, represented by the sum of the degrees on either side of the true zero. dr dl rl

Then the required resistance =

d + dl

If it be necessary to use what is known as an

ferred zero for this test, that is, if the suspended system of the galvanometer is set to such an extreme

that, normally, the spot is entirely off the scale, then we require to know R, which is the combined resistance of the battery E and the shunted galvanometer G, and 2 (R+ r) (R + rl) – R

x =

(R+r)+(R+rl)

Kempe's Loss of Current Test is comparatively simple, but necessitates the use of two galvanometers, one at either end of the line A B, Fig. 73. The testing battery E is of low resistance, and is connected through the galvanometer G and compensating battery El to the line at the point A. El is a low resistance battery of one to two cells, to balance the earth current if such exist in the line, and the testing battery E should be connected in the same direction. s is a shunt across the terminals of El for its final regulation, so that it exactly balances the earth current. To effect this adjustment, E is first disconnected, and the corresponding terminal of G put to earth. s is then adjusted until no deflection is obtained, when E is again connected up, as shown.

G and G1 are Thomson reflecting galvanometers provided with low resistance shunts. Simultaneous observations are made on G and G1, which are then connected up under the same conditions with a standard cell and a resistance, which, in combination with the galvanometer resistance, will allow the passage of, say, one milliampère. The resulting deflections are noted and divided into the deflections previously obtained, when the galvanometers were connected to the cable. The quotients, which we will call C and C1, represent the respective currents which flowed through the galvanometers G and G1 when connected as in Fig. 73 above. Then the required resistance x between the extremity A and the Cr C1 (R+ Rg) where R is the original

C C1

fault, x = conductor resistance of the line between the points A and B, and Rg the resistance of Gl. r is the resistance on the far side of the extremity A through earth, and can be ascertained by disconnecting the batteries E and El and galvanometer G from the line and earth, and connecting them instead through an adjustable resistance which is varied until the original deflection is reproduced, then its value is the same as r.

Clark's Fall of Potential Method is illustrated in Fig. 74, where B D is a length of cable having a fault at the point C. A B is a length of good cable of resistance

R, joined in series with it. E is a battery of constant E.M.F. connected between the point A and earth, and G is a galvanometer of the d'Arsonval type in series with a resistance r, such that their combined resistance is appreciably greater than that of the line under test. G is first connected between the points A and B, D being insulated, and the deflection due to the fall of potential from A to B noted; we will call it d. G is then disconnected, and connected instead as shown in the figure between the point B and earth, and the resulting deflection dl is also noted. The galvanometer G is then removed, and set up under exactly the same conditions as to scale distance, etc., at the point D, and connected between the point D and earth, the third deflection d2 being noted, then the resistance between the point B R dl d2 d

and the fault, x =

It is quite possible, with a D'Arsonval galvanometer, to shift it in this manner without altering its constant, but, if such an instrument be not available, or if the extremities B and D are so far apart that independent tests with various apparatus have to be taken, involving the use of two distinct galvanometers, then the test is rendered somewhat more complicated, as the instruments must afterwards be connected in series with a high resistance and standard cell, and their "constants" noted in terms of the E.M.F. of the standard cell, thus providing a means of comparing the respective deflections d, dl, and d2, on the same basis, and reducing them all to the values they would have attained had they all been produced upon the same instrument, as described above.

Siemens' Equal Potential Method is somewhat similar to the foregoing, and is illustrated in Fig. 75, where A B is a line having a fault at C. The mode of procedure is as follows:-The battery E, of constant E.M.F. is connected as shown between the extremity A and earth, the point B being insulated. A galvanometer (preferably D'Arsonval) is then connected between A and earth, and the deflection d noted. The galvanometer is next connected between B and earth, and a second deflection dl, due to the potential at B, and consequently at the point C, is obtained. The battery is then dis

connected from A and connected instead between B and earth, the same pole being connected to the line as in the first instance; then, the galvanometer being con

nected between the point A and earth, the resistance r in the battery circuit is adjusted until the second deflection dl is reproduced. The galvanometer is then again connected between the point B and earth, and a third deflection d2 is obtained; then, taking L as the total length of the line A B, the distance between A and dl

the fault at C, x = L

d

Of course, if separate galvanometers, or a single instrument with a variable constant be employed in making this test, the same remarks as to the reduction of the values of d to a common comparative basis apply as in the preceding case.

Siemens' Equilibrium Method is illustrated in Fig. 76, and its principle consists in so arranging two independent electromotive forces, one at either end of the line, that the potential at the fault is zero, and no current in consequence will flow to earth at that point.

A B, Fig. 76, represents the line under test with a fault at the point C. E and El are two batteries with their opposite poles connected to the extremities of the line through the resistances r, rl, r2, and r3 respectively, of which rl and r2 are equal fixed values, and r, r3 adjustable. Two galvanometers giving the same con