This strain on the blocks, which always exists in those sets in which a resistance is thrown in by removing a plug, tends to separate or loosen them and often to warp the hard rubber upon which they are mounted. Another advantage of the decade plan is that it permits obtaining a succession of values by means of sliding contacts or dial switches, a method which is becoming deservedly more appreciated. [Illustration: Fig. 575.--Leeds and Northrup dial Wheatstone bridge. Rotating switches are used instead of plugs, which permits quicker adjustment of the resistances, adapting it to rapid working. The ratio coils are arranged as in fig. 568. There are four dials which form the rheostat. The units dial contains 9 one ohm coils; the tens dial, 9 ten ohm coils; the hundreds dial, 9 one hundred ohm coils, and the thousands dial 9 one thousand ohm coils. The values of the ratio coils are 1, 1, 10, 10, 100, 100, 1,000, 1,000, 10,000, 10,000.] Ques. What is the difference between "plug out" and "plug in" types of resistance box? Ans. In the plug out type, resistance is put in the circuit by removing plugs, as in fig. 565; in the plug in type, resistance is put in the circuit by inserting plugs as in figs. 570 and 571. [Illustration: Fig. 576.--Queen Acme portable testing set. It consists of a Wheatstone bridge, with reversible arms, battery of four dry cells, D'Arsonval galvanometer, battery and galvanometer keys. There are sixteen resistance coils, having a combined resistance of 11,110 ohms. Each bridge arm is provided with three coils of 1, 10, 100 ohms, and 10, 100, 1,000 ohms respectively. The commutator admits of a ratio of 1 to 1,000 on either bridge arm, giving the set a theoretical range from .001 of an ohm to 11,110,000 ohms. For resistances above 1,000,000 ohms, the normal battery power must be increased. The contact keys are located as shown. The battery key has single contact, but the galvanometer key has double contact; depressing it closes the galvanometer circuit, and releasing it short circuits the galvanometer, bringing the latter quickly to rest.] Testing Sets.--For convenience in testing, a combination of the instruments used is put up in a neat and substantial case, and known as a testing set. There are innumerable forms of testing set, a few of which are shown in the accompanying illustrations. The usual combination is a Wheatstone bridge, galvanometer, battery and necessary keys and connections.