3 2 



ELEMENTARY LESSONS ON [CHAP. vi. 



359. Resistance Coils. Wires of standard resist- 

 ance are now sold by instrument makers under the name 

 of Resistance Coils. They consist of coils of german- 

 silver (see Art. 349) (or sometimes silver-iridium alloy), 

 wound with great care, and adjusted to such a length as 

 to have resistances of a definite number of ohms. In order 



to avoid self-induction, 

 and the consequent sparks 

 (see Art. 404) at the 

 opening or closing of the 

 circuit, they are wound 

 in the peculiar manner 

 indicated in Fig. 131, 

 each wire (covered with 

 silk or paraffined -cotton) 

 being doubled on itself 



before being coiled up. Each end of a coil is soldered 

 to a solid brass piece, as coil i to A and B, coil 2 to 

 B and C ; the brass pieces being themselves fixed to a 

 block of ebonite (forming the top of the "resistance 

 box "), with sufficient room between them to admit of 

 the insertion of stout well-fitting plugs of brass. Fig. 

 132 shows a complete resistance -box, as fitted up for 



Fig. 132. 



electrical testing, with the plugs in their places. So 

 long as the plugs remain in, the current Hows through 



