[Illustration: 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. [Illustration: Fig. 568.--Ratio coils of Wheatstone bridge. Almost every box intended to serve as a Wheatstone bridge is furnished with a set of coils which forms the arms of proportion or ratio arms of the bridge. There is a choice of several different ways of arranging these coils. The figure shows the simplest arrangement, which is employed in boxes not intended for the highest accuracy. The required ratio, as for instance 1:100, is obtained by withdrawing a plug from each arm A and B. Ratios 1/1, 1/10, 1/100, 10/100, etc., or 1,000/1, 1,000/10, 1,000/100, 100/100, etc., are obtainable in this manner. This simple arrangement is open not only to the objection that the contact resistance of the plugs which remain in is always included with the resistance unplugged, but also to all other objections to be urged against the use of many plugs where a few will do. The method has the limitation that it is not possible to reverse the arms of the bridge, that is, to transpose the arms A and B.] Ques. Why should the battery key be depressed before the galvanometer key? Ans. To avoid the sudden swing of the galvanometer needle, which occurs on closing circuit in consequence of self-induction. Ques. How is it known whether too much or too little resistance be unplugged? Ans. The galvanometer needle will be deflected to one side for too much resistance, and to the opposite side for too little resistance.