contacts are negligible because only two plug contacts enter the circuit, and with an even ratio, it is only the difference in the resistances of the two plug contacts that can affect the result. In measuring any of the ratio coils while in the box it is only necessary to connect to the bar C and to either the bar A or B and plug in the coils to be measured.] [Illustration: Fig. 573.] [Illustration: 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.