THE MEASUREMENT OF RESISTANCE 167 



method is slower in its application than the null method, and 

 the arms of the bridge are more likely to be overheated. The 

 battery e.m.f. must remain constant. 



Examples of Arrangements of Bridge Tops. A very satis- 

 factory form of bridge top is shown in Fig. 90. All the connec- 

 tions made in setting up the instrument are outside the box, the 

 wires being attached to the appropriate binding posts. 



From the plan of the top, it will be noted that there are three 

 gaps at 1', 2', 3' each marked Inf ; removing the plug from any 

 one of these gaps breaks the circuit. The 10,000-ohm coil may 

 be used in either the balance or the rheostat arm. Reversal of 

 the bridge arms is accomplished by placing X at 3' instead of 1', 

 the galvanometer terminal being changed also. By removing 

 the plugs at 1' and 2' and 3', the coils are divided into two indepen- 

 dent sections; occasionally this is very convenient where the 

 box is used for general laboratory purposes. 



Another design employing the series arrangement of coils is 

 shown in Fig. 91. Here the connections are permanently made 

 inside the box. The reversal of the ratio arms is effected by 

 changing the plugs from positions 1' to 2'. 



In Fig. 92 is shown a bridge employing the dial arrangement 

 for the rheostat arm. The unknown resistance is inserted at 

 either 1 or 2 according to the ratio desired. The dial bridge in 

 this form is not recommended because the condition of the top 

 is not apparent at a glance and there is also difficulty in clean- 

 ing it. 



A dial bridge with sliding in place of plug contacts in its 

 rheostat arm is shown in Fig. 93. 



In this bridge the ratio coils are arranged as suggested by 

 A. Schone 3 (see Fig. 94). There are two coils of each of the 

 following denominations, 1, 10, 100, 1,000 ohms. All the coils 

 have one terminal attached to the central copper bar which is 

 inside the box, the other terminals being connected to the plug 

 blocks. Two plugs are ordinarily used. When inserted as 



shown, -^ = -IT0-. The advantages of this arrangement are the 



ease with which the arms may be reversed, the reduction in the 

 number of plug contacts, and the possibility of obtaining the 

 the same ratio by using different coils of the same denomination, 



M 



thus giving a check on the value of -. 



