Figs. 573 and 574.--The Leeds and Northrup decade. The object of this arrangement is to reduce the number of coils required. In [fig. 573], the 1, 3', 3 and 2 are connected in series. Let the terminals of the 1 ohm and 2 ohm coils be numbered (1), (2), (3), (4) and (5) ([fig. 573]). The current enters at point (1) and leaves the coils at the point (5), traversing 1, 3', 3, 2 = 9 ohms in all. If this series be multiplied by any factor n, then n (1 + 3' + 3 + 2) = n 9 ohms. It will be seen that if the points (1) and (5) be connected, all the coils are short circuited, and the current will traverse zero resistance. If the points (2) and (5) be connected, the 3', 3 and 2 ohm coils will be short circuited and the current will traverse 1 ohm. By extending the process so as to connect two and only two points at a time it is possible to obtain the regular succession of values n (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the last being obtained when no points are connected. [Fig. 574] shows Leeds and Northrup's method of connecting these points two at a time with the use of a single plug. The circles in the diagram represent two rows of ten brass blocks each. To the first two blocks at the top of the rows, the points (5) and (1), [fig. 573], are connected; to the second two, the points (2) and (5) are connected, etc., no points being connected to the last pair of blocks. Hence, if a plug be inserted between blocks 1 and 5, [fig. 575], the points (1) and (5) of diagram [fig. 573] are connected, giving the value of 0, if between the blocks 2 and 5 the points (2) and (5) are connected, giving the value 1, and so on. The value 9 is obtainable when the plug is in the last pair of blocks, which have no connections. [Fig. 572] shows a top view of the blocks of a simple decade constructed upon this plan.
Ques. What other advantages are gained with the decade arrangement?
Ans. The single plug used with each decade is never out of use, being either in the zero position or set on some value, and is therefore not easily lost by being laid aside. The use of only one plug in a decade makes it easy to ascertain that the plug is making good contact as only one block in a row is plugged at a time, the other blocks are not kept under a strain by having plugs forced tightly between them.
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.
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?