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whether K be open or closed. When the required result has been obtained, the resistance of the battery

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Thomson's method is indicated in Fig. 34, where E represents the battery under test as usual, G the galvanometer,

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R an adjustable resistance, and s a shunt across the terminals of the battery. In order to carry out this test, we require, in the first instance to know the resistance in ohms of the galvanometer G and the shunt s respectively. The mode of procedure is as follows: The connections being made as in Fig. 34, the galvanometer deflection is noted, and the shunt s is then removed from the circuit, R being simultaneously increased to Rl such that the resulting deflection on the galvanometer is the same as before, then the resistance of the battery E


R + G The internal resistance of a battery when it is supplying energy to an external circuit, differs from its internal resistance when idle, owing to the chemical reactions set up within the cells, and other causes. In many cases it is necessary to ascertain this internal resistance of the cells when at work, and Fig. 35 represents a method of effecting this. E represents the battery under test, supplying current to the external resistance R under control of the key

= S

or switch K; s is a shunt across the battery terminals, the value of which in ohms should be between that of the battery itself and double its value, but neither more nor less for satisfactory working. It is inserted or cut out of

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circuit at will by the key Kl. C and G are a condenser and galvanometer respectively, whose presence in the circuit is controlled by K2.

The modus operandi is as follows: Close K, and, if the current be steady, open it again and close K2, noting the resultant deflection d. Now open K2 and again close K until the galvanometer reaches zero, when again open K and close Kl and K2 in rapid succession, taking care to note the second deflection d1, then the resistance of the battery


dl E =S

dl. Of course, this method does not actually determine the battery resistance whilst working, but immediately after the cessation of work; for this reason the keys Ki and K2 should be manipulated as soon as possible after K has been opened.

If the external load be fairly constant, the same connections will answer for an actual test whilst working. Το effect this, depress K permanently, or, if necessary, cut it out of circuit. Close K2 and note the resultant deflection d, then, with K2 still closed, close Kl, which will give rise to a deflection in the reverse direction d1, then the battery resistance E

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The quantity ile may be neglected if the value of R be

high, i.e., if the resistance of the external circuit be large, whilst, if the battery be idle, and R be in consequence, totally disconnected it disappears altogether and we have


E = s

d - di Next on our list comes an all-important test which will invariably be met with in any electrical undertaking, and to which we have already several times referred, viz.,

(4) Resistance Measurement by the Wheatstone Bridge Method.The connections for this test have already been indicated in the previous pages by Fig. 12, a familiar diagram utilised in explaining the principle of the Wheatstone bridge, but it is reproduced in more practical form in Fig. 36.

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The apparatus required for this test are a moderately sensitive galvanometer, the previously described P.O. pattern of Wheatstone bridge for simplicity's sake (including keys) and some half-dozen cells of a suitable type, such as the Leclanché or Daniell. Connect up as shown in Fig. 36, x being the unknown resistance which it is required to measure. If possible, it is advisable for the operator to

b c


gain a rough insight as to the probable resistance of a before commencing the test, as matters are thereby simplified by the preliminary manipulation of the bridge plugs. Where such approximate information is at hand, and the probable result is comparatively low in value, arrange for a large resistance in a, and a small one in b, and vice verså if the probable result be high. If moderately in the centre range of the instrument, equal resistances may be plugged in the proportional arms. The INF plugs must not be withdrawn for this test. Having unplugged the requisite proportional resistances, arrange the adjustable arm, if possible, for an approximate result, remembering that the unknown resistance x =

Now depress the battery key K1 first, and subsequently the galvanometer key K, when, if a deflection be obtained, remove or insert plugs in c until no deflection results when K is depressed, K1 being kept down all the while.

In cases where no idea exists as to the probable result, a balance must be obtained on the galvanometer by manipulation of the plugs, the best plan being to start with the thousands, and obtain a reversal of the deflection; then work down to the hundreds, tens, and units, obtaining final adjustment over the range of the latter. If no reversal can be obtained by any arrangement of plugs, the unknown resistance ¿ is probably beyond the range of the bridge, its resistance being very high or extremely low.

When a case occurs in which it is impossible to obtain an exact balance on the galvanometer, the addition or subtraction of one unit in the adjustable arm giving two different deflections in opposite directions with regard to the zero point, the following formula for calculating XC will be found efficient :

Let a and 6 be the values of the proportional arms,
Let c be the lower value of the adjustable arm.
Let cl be the higher value of the adjustable arm.
Let d be the deflection resulting from c.
Let dl be the deflection resulting from cl.

Then x =

d + dl

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The same method of resistance measurement is also applicable to the metre bridge, previously described. This method, although not quite so accurate in its results as the preceding one, is nevertheless very handy, and suitable for workshop practice. Its accuracy mainly depends on the uniformity of the actual slide wire considered with regard to its ohmic resistance.

The connections are represented in Fig. 37.

The apparatus required is the same as in the preceding test, with the exception of the Wheatstone bridge, which is in this case replaced by the metre bridge. As will be seen from the diagram, the standard resistance a, and the resistance to be measured, x, are inserted in gaps 1 and 2. In practice it is as well, if circumstances permit, to make a approximately equal to x, thus having the equivalent of equal values in the proportional arms of the Wheatstone bridge. This arrangement will enable the slider to be adjusted somewhere near the centre of the slide wire. It will be noticed in Fig. 37 that an extra key for the galvanometer is added to take the place of that which in the P. O. pattern of Wheatstone bridge is usually fitted on the ebonite top of the instrument itself. The battery key is provided for by the action of the slider itself, which makes and breaks contact with the slide wire at will.

The method of conducting the test is precisely similar

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