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G oscillates slightly over zero before moving rapidly over the scale under the influence of the returning cable current.
In making this test care must be taken to apply the same pole of the battery to the bridge circuits as was employed to neutralise the cable current, and also to make the necessary observations on the bridge galvanometer G at the same instant that zero is noted upon G1, as, from that moment, the fault will commence to polarise again in the same direction as before, and must be subjected to the further neutralising pole of the testing battery before any more observations can be taken on G.
Lumsden's Method has a similar object in view to that embraced by the foregoing, viz., the preliminary neutralisation of the polarisation current due to the fault. It is performed as follows :—The fault is first cleaned by the application to the end of the line of a zinc current from some 60 to 100 cells, the opposite pole being earthed. This application is continued for a period of from 10 to 12 hours, according to the requirements of the case, the direction of the current being reversed from time to time for a brief interval. The line and instruments are ther connected up, as shown in Fig. 71, and a rough measurement of the resistance is made and left unplugged in the variable arm C of the bridge. A positive current from a battery consisting of some three cells to every 100 ohms resistance is then applied for the space of a minute or so, the effect being to coat the conductor at the fault with chloride of copper. Both keys are then depressed, and the plugs manipulated in such a manner as to keep the needle of the galvanometer G (which must be of the same type as that employed in the preceding test) at zero. The needle will be continually but slowly varying, and must be maintained approximately at zero by regulating C, until, at a certain point, the needle will be rapidly deflected, thus indicating the fact that the pure copper of the conductor has been exposed at the fault. The value in C at this moment gives the true resistance, in the absence of polarisation currents. The operation should be re peated and an average of the results taken ; it will, however, not be necessary to apply the negative current for so long a period in the second case, from 20 to 30 minutes
usually proving ample for the purpose. If the fault be near the extremity of the line attached to the bridge, the necessary time for the manipulation of C may be
gained by the introduction of a known resistance between it and the bridge, thus lessening the current effect and increasing the time interval. The value to be given to this resistance must be determined experimentally, and it must, of course, be deducted from the ultimate results obtained in the test.
Mance's Method has for its main object the elimination of errors due to earth currents which are distinct from the polarisation currents set up at the fault, in that they are the result of a difference of potential existing between two points on the earth's surface occupied respectively by the available extremity of the cable and the fault.
The connections for the test are the same as indicated in Fig. 71, and the method consists in making the resistance measurement in the usual manner, first with two equal values r such as the 100 ohm coils in the proportional arms A B of the bridge, and secondly with two other equal values rl such as the 1,000 ohm coils, for example. Let R and Ri be the respective values unplugged in the variable arm C of the bridge in either of
the above cases, and let Re be the resistance of the testing battery, then the required resistance
R (2 Re + r) RI (2 Re + rl)
(R + r) The first measurement with the proportional coils r unplugged is continued until it becomes steady, and the galvanometer is then short-circuited for an instant or cut out of circuit by its key, whilst the proportional coils are changed to rl, when a fresh value in C, usually larger than R, will be obtained on balancing.
Jacob's Deflection Method dispenses with the necessity for using a Wheatstone bridge in the actual test made on the cable, the speedy manipulation of its plugs being somewhat inconvenient.
The instruments are first connected as indicated in Fig. 72, and consist of the testing battery E, a battery reversing switch S, and a Thomson reflecting galvanometer G, with a reversing key K, short-circuit key Ki, and low resistance shunt s. The suspended system of the galvanometer G is turned so that it has a false zero at one extremity of its scale instead of in the centre, as is usually the case. The battery switch is first closed, and that side of the galvanometer key K depressed which tends to give a deflection over the range of the scale, the resulting deflection d being regulated as regards its convenient dimensions by the shunt s. The keys are then opened, and the battery current reversed by means of its switch, the other side of the galvanometer key K being depressed in order that the second deflection di may be in the same direction as the first. Owing to the earth current, these two deflections d and dl will have different values, and the shunt s must be manipulated until they are both conveniently within the range of the scale. A series of measurements is then taken with either pole of the battery, the key Ki being employed to check the swing of the galvanometer, and so tend to rapid working. An average of d and di is obtained from the series, and the line and earth are then disconnected and replaced by a set of adjustable resistances. The same operations are then repeated, and the value of the resistances adjusted to r and rl, such that the deflections d and dl are reproduced.
The deflections d and dl are, of course, represented by the sum of the degrees on either side of the true zero.
dr t. dl rl Then the required resistance x =
d + dl If it be necessary to use what is known as an “in
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
(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 Ci, represent the respective currents which flowed through the galvanometers G and G1 when connected as in Fig. 73 above. Then the required resistance between the extremity A and the
Cr - Cl (R + Rg) where R is the original fault, x =
C - CI conductor resistance of the line between the points A and B, and Rg the resistance of Gl. p 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.