CHAPTER III 



Brass Tube 



RESISTANCE DEVICES 



The resistance devices used in electrical measurements may b<> 

 divided into two groups, resistance boxes and rheostats. 



A resistance box is a device by which the resistance of a circuit 

 may be altered by accurately known amounts. It contains an 

 aggregation of coils, each coil having a definite and known 

 resistance, and the construction is such that the coils may be 

 connected in various combinations so that any required resistance, 

 up to the full capacity of the box, may be 

 obtained. 



The term rheostat is applied to the de- 

 vices commonly used for varying the re- 

 sistance of the circuit where regulation of 

 currents and absorption of power are con- 

 cerned. The circumstances under which 

 rheostats are used render it unnecessary 

 that the magnitudes of the variations in 

 resistance be known. 



Resistance Coils. Formerly it was the 

 universal practice to wind resistance coils 

 on wooden bobbins, but, in the better class 

 of work, these bobbins have been replaced 

 by metal spools (see Fig. 57). A layer of 

 shellaced silk which is dried out by baking before the coil is 

 wound serves to thoroughly insulate the wire from the metal 

 spool. 



Non-inductive windings are always employed. The wire is 

 arranged in a bight before it is wound upon the bobbin and the 

 two wires are kept side by side in the coil. 



If possible, the winding is concentrated in a single layer, for 

 as all the heat must be dissipated through the surfaces of the 

 coil, one wound several layers deep with a large wire is not supe- 

 rior to one wound with but a single layer of small wire. 



128 



Binding 

 Silk 



"nfiesistance 

 Wire 



* Brazed 

 >pper 



Soldered 



FIG. 57. Section of re- 

 sistance coil. 



