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this description often occur in which a wire has become broken and the two severed ends are held together by the surrounding insulaticn which remains undainaged. Other cases of intermittent disconnection frequently occur with overhead telephone wires which break, and, remaining suspended over adjacent wires, are brought into intermittent contact by the agency of the wind.
The National Telephone Company adopt the above simple system of circuit testing universally for daily line tests and fault localisation, each exchange being provided with a detector outfit and special switching arrangements, by means of which it can instantly be connected with any required line at will.
A very good method of determining an intermittent disconnection on a dynamo, or any local electrical apparatus which
be accessible to the operator, is as follows: The section in which the fault lies having been localised as before described, the leads a and b are connected to its extremities, and, the galvanometer being carefully observed, a sharp blow is struck the object under test, with a wooden mallet or other object calculated not to injure the surface in any way. The vibration thus set up in the mass of winding will frequently separate two broken ends if they be in contact, or bring them together if they be apart, a resultant flick being observed as before on the part of the galvanometer needle, thus effectually determining the existence of a fault.
The methods of continuity or circuit testing described above are, of course, very rough, and give no indication as to the ohmic resistance of the circuit under test; a fault may thus exist which is of high resistance but sufficiently small to indicate a complete circuit when subjected to the detector test. In such case there is nothing for it but to measure the resistance of the circuit by the methods to be described later; the excessive value then obtained will be a sufficient indication of the existence of a fault.
The familiar law of Ohm is almost too well known to need repetition here, but, as it concerns the principles of all electrical testing, it is herewith submitted to the reader once again. The current flowing in any circuit is equivalent to the electromotive force in that circuit divided by the total resistance thereof, or, as it is more commonly ex
E pressed, C the quantities involved being in terms of
R their respective practical units, to wit, the ampère, volt, and ohm.
Thus, if we refer to Fig. 30, which represents a simple circuit, Ohm's law tells us that the current in ampères flowing from, say, the positive pole of the battery E, through a, the galvanometer G, b, and the battery itself, back to that positive pole, is equal to the E.M.F. in the circuit which, as in this case there is no other source of current, will be the E.M.F. of the battery E (in volts), divided by the total resistance of the circuit (in ohms), which latter will include the resistance of the galvanometer G, and connecting leads a and b, together with the internal resistance of the battery E.
It is obvious, therefore, that in order to analyse the condition of this circuit when a current is flowing through it, we must first arrive at the respective resistances of the galvanometer G and battery E, and we will now proceed to discuss one or two methods of:
(2) Galvanometer Resistance Measurement.—There are two very good methods of taking the resistance of a galvanometer, known respectively as the “Half Deflection and "Equal Deflection" methods. The galvanometer whose resistance is to be measured, is connected as shown in Fig. 32, where G is the galvanometer, E the testing battery of one or more cells, according to the sensitiveness of the instrument under test, and R an adjustable resistance, such, for instance, as the adjustable arm of a Wheatstone bridge. If the galvanometer be of a sensitive type, it will be as well to introduce a short-circuit key across its terminals, and, if considered necessary, an ordinary circuit key, or switch may be included in the circuit, but these accessories
have been omitted in the figure for the sake of clearness. The half deflection method consists in making R a certain fraction of what the galvanometer resistance is likely to be, and noting the deflection obtained on the galvanometer G when the circuit is completed. Now, increase the value of R to R1 such that the deflection obtained through it is equal to half the original deflection, then the resistance of the galvanometer will be equal to the increased resistance Ri less twice the original resistance R, or, expressed as a formula, G = R1 - 2R. .
The connections for the equal deflection method are precisely those represented in Fig. 32 for the foregoing test with the one addition of a shunt, s, across the terminals of the galvanometer. The battery in this case must be of low resistance. Note the galvanometer deflection as before, with the shunt, 8, connected ; then disconnect 8 and increase R to R1, such that the same deflection is obtained as
RI-R before, then resistance of galvanometer (G) = 8
R A still more reliable method of taking the resistance of a galvanometer is that known as the Wheatstone bridge method of resistance measurement, which has already been alluded to, and will be described later in detail. It involves the use of a second galvanometer in taking the test. If the galvanometer under test be of the reflecting type previously described in these pages, its suspended system should either be temporarily removed or suitably supported, in order to protect it from any injury which might accrue from the passage of the testing current. The temperature in the immediate vicinity should also be taken at the time of making the test, as the resistance varies with a variation in temperature ; in the case of copper it increases about 0.21 per cent. per 1 deg. F. rise in temperature, or about 0.38 per cent. per 1 deg. C. This quantity is known as the temperature co-efficient, and it varies with different metals, its value for copper being comparatively high. For this reason, copper is seldom used nowadays in the winding construction of electrical instruments and apparatus, but is replaced by such alloys as manganin and platinoid, the temperature co-efficients of which are so low as to be almost negligible.
(3) Battery Resistance Measurement.—Here, again, we have a still greater multiplicity of available methods. It is obvious, of course, that we cannot measure the internal resistance of a current-producing cell as we can the resistance of a coil, for example, because the current from the cell itself would oppose or aid the current from the testing battery, as the case may be, and thereby introduce considerable error into the result. In the case of a large number of cells, all of equal resistance and E.M.F., such as a battery of accumulators, for instance, the resistance can be taken by the Wheatstone method, by splitting the battery up into two halves and opposing them to one another, thus taking the combined resistance which will, of course, be equal to that of the whole battery in series ; if necessary, one or two extra cells may be allowed in one half to supply the testing current.
When the above mode of procedure is not possible, it is necessary to adopt one of the following methods :
What is known as the reduced deflection method consists in connecting up as in Fig. 32; in this case the galvanometer should be of low resistance, such that its resistance added to that of R will be a fraction of the resistance of the battery E. Note the initial deficction as before, we will call it d, then increase the value of R to Ri and note the lesser deflection dl, then the resistance of the battery
RL dl Rd
Matters will be considerably simplified if Ri be made of such value that the second deflection, dl, is exactly half the first, for in such a case E = RI (2R + G).
A still simpler modification consists in shunting the galvanometer by a short stout wire, such that the combined resistance of galvanometer and shunt is a negligible quantity. The battery and galvanometer are first connected up per se, and a deflection obtained and noted ; the resistance R is then introduced, and regulated to such a value that the original deflection is halved, then E = R, from which it will be seen that the required internal resistance of the battery is read off directly from R.
Mance's method of measuring the resistance of a battery is represented diagrammatically in Fig. 33, where a, b, and c represent the proportional and adjustable arms respectively, of a Wheatstone bridge, G a galvanometer, E the battery under test, and K an ordinary circuit key or switch. The method consists in adjusting c until the manipulation of
the key or switch K has no effect upon the galvanometer G, or, in other words, c is brought to such a value that the deflection of the galvanometer needle remains the same