T3^> PHYSIOLOGICAL PHYSICS. [Chap. xin. 



By a resistance foox (Fig. G9) a much greater 

 resistance may be interposed in a circuit, and as easily 

 graduated. It is made of a series of bobbins on which 

 are coiled various lengths of insulated wire. The coils 

 are placed in a box, and the two ends of the wire of 

 each bobbin are connected with two different plates of 

 brass fitted on to the ebonite lid of the box. ABC at 

 the side of the figure show how the coils are con- 

 nected with two separate brass plates. Each coil 



Fig. 69. -Resistance Box. 



offers a certain amount of resistance, which is marked 

 in ohms on the lid of the box between the two brass 

 plates to which it is attached. There are also binding 

 screws attached to the lid. Suppose the wires from a 

 battery to be attached to the screws, the current would 

 require to traverse all the coils in the box, and would 

 thus encounter a resistance equal to the total offered 

 by the coils. But the brass plates on the lid are so 

 arranged that thick brass plugs, may be made to fit in 

 between them, so as to connect them. "Where two 

 brass plates were so connected the current would 

 not traverse the coil attached to them, but would 

 pass straight across from one plate to another by 

 means of the plug, and the resistance of that coil 

 would therefore be put out of the circuit. Suppose 

 all the plugs were in, the current would traverse none 

 of the coils, but would pass straight from one binding 

 screw through plates and plugs to the other, and 

 owing to the thickness of plates and plugs would 

 encounter practically no resistance in the box. 



