For instance, if A and C be, as in [fig. 565], 10 ohms and 100 ohms respectively, and B be 15 ohms, D will be (15 × 100) ÷ 10 = 150 ohms.
Fig. 566.--Diagram showing usual arrangement of resistances in arms of Wheatstone's bridge. In practice the bridge is seldom or never made in the lozenge shape of the diagrams, figs. [564] and [565], these being given merely for clearness. The resistance box of [fig. 554] is, in itself, a complete "bridge," the appropriate connections being made by screws at various points. The letters in the above diagram correspond with those in figs. [564] and [565], and the three figures should be carefully compared.
As constructed, Wheatstone bridges are provided with some resistance coils in the arms A and C, as well as with a complete set in the arm B. The advantage of this arrangement is that by adjusting A and C, the proportionality between B and D can be determined, and can, in certain cases, be measured to fractions of an ohm. In [fig. 565] resistances of 10, 100, and 1,000 ohms are included in the arms A and C.
Fig. 567.--Standard resistance box and Wheatstone bridge. This pattern is a modification of the Anthony form of bridge. All the resistances are wound upon metal spools. The bridge ratio coils are 1, 10, 100, 1,000, 10,000. The rheostat coils are arranged in five rows, of ten coils each. The ordinary decade plan (explained in [fig. 570]) is followed. The coils may be joined in series in multiple, or in any combination of series and multiple. The coils may thus be checked against each other in many combinations. For instance, all the ten ohm coils taken in parallel may be compared with any one ohm coil. The precision of adjustment is said to be 1/20th of 1% for the coils of the tenth ohm series, and 1/50th of 1% for the coils of the rheostat. The ratio coils are certified to be like each other to within 1/100th of 1%. The box is supplied with battery and galvanometer keys of substantial construction.
Ques. Describe the method of testing with the bridge.
Ans. [Fig. 567] illustrates the general arrangement of resistances to be found in an ordinary bridge. The connections are made as shown. In testing, first depress the battery key, then tap the galvanometer key. This should be repeated adjusting the resistances till no deflection is obtained. The resistance then in the arm B × (C ÷ A) will give the value of the unknown resistance.
