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ances of very low value consist in what are known as difference of potential methods.
Referring for a moment to Fig. 41, it is a well-known fact that if we take a length of wire, cable, or other conductor, A B, and pass a current through it from the battery E in the direction indicated by the arrow heads, viz., from A to B, there will be an E.M.F., or, as it is often termed, difference of potential between the points A and B equal to the E.M.F. of the battery E, the resistance of the connecting wires being regarded as negligible; the potential may, in fact, be taken as falling from a maximum A a at A to zero at B. This fall of potential along the wire A B is directly proportional to its resistance, and it is possible for us to determine the fall, or difference of potential between any two points along the wire A B by the application of a pair of leads connected to a suitable galvanometer. We can, for example, compare the fall of potential between the points A and b with that between 6 and B, and, since this fall is directly proportional to the respective resistances of these sections, we can, by obtaining a value on the galvanometer for each of the two sections A b and b B, also compare their respective resistances.
The practical method of applying this principle to low resistance measurement is illustrated in Fig. 42, where A B is a standard homogeneous wire of low resistance, and B X the resistance which it is required to
G is a galvanometer whose connecting leads a b can be applied at will
, either to the points A B or B X. E is the testing battery of one or two cells capable of yielding a constant current (an accumulator answers
very well), and K is a simple circuit key for controlling the current supply. A B may be provided in practice by the slide wire of a metre bridge, the connection a being made through the slider, a proceeding which simplifies matters considerably, as will be shown presently. The method consists in applying the leads a b to A B und B X respectively, the key K being closed in each case. The two deflections d and dl are duly noted, then the resistance of A B: the resistance of B X : : d: dl.
If we connect a to the slider, and manipulate the latter along the wire A B, until a point A' is found, at which the deflection d equals dl, then A' B equals B X.
It will be readily understood that in dealing with very low resistances contact will be an important matter, and so it is, in the battery circuit, i.e., at any point in the circuit bounded by A B X K and E. In the galvanometer circuit it makes no difference to the test.
In order to eliminate the introduction of any extra resistance into the current circuit through bad contacts, the latter should be well made through massive terminals, or even, if need be, sweated. A better plan still is to make the contacts at A B and X through mercury cups, the ends of the wires being first well cleaned with emery paper and subsequently amalgamated by rubbing with nitrate of mercury before their immersion in the cups. The galvanometer G for this test should be fairly delicate, in order that accurate adjustment of the slider may be obtained. One of Thomson's reflecting galvanometers, having a resistance of some four or five thousand ohms, unswers very well, but should be protected by a shunt until very nearly balanced, when the final adjustment can be made with the shunt cut out of circuit. If it be available, a differential galvanometer answers admirably for this test, the two windings being connected to A B and B X respectively, a being adjustable as before, is manipulated until no deflection results on the galvanometer, then A' B equals B X, as before.
Messrs. Nalder Bros. have applied this principle of low resistance measurement to an instrument which combines practically all the apparatus required upon one base. It is illustrated below, whilst Fig. 43 represents a working diagram of the apparatus.
Low Resistance Measurer by Nalder Bros., designed on the potentiometer principle. N.B.- This illustration represents the instrument in circular form, whilst the diagram, Fig. 43, is taken from the longitudinal form of apparatus.
A B, Fig. 43, is a standard homogeneous slide wire, connected by means of massive terminals in series with the resistance to be measured, X; a storage cell, E, capable of yielding a constant discharge of some 10 ampères, and an adjustable resistance, Ř, which is manipulated until the above current, or its approximate value, as indicated on an ammeter connected in circuit, but not shown in the diagram, is flowing through the circuit. S is an adjustable slider, connected by means
of a 'bus bar with one winding of a small horizontally pivoted differential galvanometer G, through the regulating resistances x, y, and 2, which are controlled by the three-way plug switch C, and, by their respective insertion in the circuit, determine the subsequent values of the results obtained in 1-2,000ths, 1-1,000ths, and 1-500ths of an ohm respectively.
The slider S registers its position of contact with the slide wire on a suitably divided scale, as in the case of an ordinary metre bridge, whilst the other winding of the differential galvanometer is brought to terminals on the base, and from them, by means of flexible leads furnished at their far. extremities with steel contact points, is led to the extremities of the resistance under test. These feelers, as we may term them, are represented in the figure by waved lines, and the contact between them and the ends of the resistance X does not affect the accuracy of the test. K is a key controlling the current circuit.
The modus operandi is as follows:-R, having been · adjusted until the requisite current of some 10 ampères flows through the current circuit when K is closed, the plug is inserted in 2, y, or 2, as th~ case may be, accord
ing to which value for the ultimate result will be nearest to the value of the resistance under test x. The slider S is then adjusted until a balance is obtained on the galvanometer G, the feelers during this operation being maintained in contact with the ends of the resistance x. The battery key K may then be released, and the scale reading carefully noted.
The scale is divided up into 100 equal parts, each of which is again sub-divided into 10 parts, and the reading indicated by the pointer when a balance is obtained will represent the resistance of x in terms of the particular fraction controlled by the plug switch C. Thus, if the plug be in y, the result may be read off the scale directly in 1-1,000ths of an ohm; if in x, in 1-2,000ths; and if in 2, in 1-500ths.
Carey Foster's method of low resistance measurement again involves the use of the indispensable metre bridge. The metre bridge proper, although it has, up to the present, been represented as possessing two gaps only, 1 and 2, Fig. 37, in the thick copper strip connections, is in reality possessed of four, the two additional onen, 3 and 4, Fig. 44, being usually bridged across by thick links of copper of similar section to the main strips themselves, and offering a practically negligible resistance to
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