Bare Wire for Resistance-coils. 211 



II. Construction of the Resistance-box and Standards. 



The resistance-box has five dials, each consisting of nine 

 coils, ranging from -^q of an ohm to 1000 ohms, and four 

 pairs of proportional arms ranging from 1 to 1000 ohms. 



The -£§ ohm coils have a length of 96 millim., the wire 

 having a diameter of "63 millim. Being so short, there is no 

 difficulty in supporting the coils. 



The wire of the 1 ohm coils has the same diameter and a 

 length of 960 millim. These coils were at first wound into 

 short spirals, and were allowed to hang freely from their 

 terminals. It soon, however, became apparent that the con- 

 volutions of the spiral required to be fixed rigidly to the 

 ebonite top. The method that has been finally adopted is to 

 screw the spiral into holes pierced in a mica plate ; this 

 plate being screwed top and bottom to a brass bar which is 

 attached to the ebonite top. 



In the coils of from 10 to 1000 ohms it is not easy to find 

 a form of bobbin on which to wind the great lengths of bare 

 wire required in such a manner as to avoid short circuits, and 

 to bring the wire in contact as little as possible with the in- 

 sulator. The form I have ultimately adopted is as follows 

 (fig. 1) : — To a central brass rod are attached, for the 10 ohm 

 and 100 ohm coils, two small brass crosses at a distance of 

 about 150 millim. apart ; to the ends of the arms of these 

 crosses are screwed serrated strips of mica, the pitch of the 

 serrations being about 1 millim. The resistance-wire is then 

 bifilarly wound into these serrations. The 1000 ohm coils 

 are of the same form, but from their greater length, 250 

 millim., it is necessary to employ a third cross in order to 

 strengthen the mica, and even then difficulty has been expe- 

 rienced owing to the mica bending under the coil-tension, 

 culminating in several cases in the breakage of the wire. 



For the 10 ohm coils wire having a diameter of *30 millim.- 

 and a length of 2400 millim. has been used. For the 100 

 ohm coils, a diameter of *15 millim. and a length of 6200 

 millim. For the 1000 ohm coils, a diameter of '075 millim. 

 and a length of 14500 millim. 



The top is of ebonite, 86 centimetres long, 28 centim. wide, 

 and 19 millim. thick. 



Instead of using divided blocks for the dials, conical plugs, 

 having a maximum diameter of 9 millim., and a taper of 1 in 

 12, fitting into conical sockets have been employed *. The 



* This method of constructing the plugs is due to Mr. R. W, Paul, 

 Proceedings of the Physical Society, 1892. 



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