and Electrical Resistance of Iron. 221 



specimens A and B were almost identical in construction, 

 differing mainly in the superiority of the insulation of the 

 later ring. They were made in the following way (see 

 figs. 2-4) :— 



A sample iron strip was chosen, and a narrow band 2 mm. 

 wide was cut by a cold chisel out of the middle, leaving a 

 specimen consisting of two parallel strips each 4 mm. in width 

 joined at one end. The mean section of this double strip was 

 determined by the specific gravity method. It was then 

 insulated with mica and covered with asbestos as shown in 

 fig. 2. 



Fig. 2. — Preliminary Stages. Half full size. 



'mica, slips /V 



- Thermometer wire ^fr \ t 



\) ttsoesco 



Fig:. 3. — Section. Full size. 



zron. CZ3 & 



mvca./ \ asbestos-insulated 



asbestos 



PC- Cherm- ivire 



The insulated thermometer-wire was next laid in the space 

 between the two iron strips, separated from them by slips of 

 mica laid crosswise, and covered over by a plain mica strip 

 (fig. 3). The whole was then rolled up on a temporary 

 wooden form into a ring of four or five turns (fig. 4). 



Thus, though asbestos was used in the core as insulation, 

 it was the mica alone that was relied upon. 



The ends of the iron specimen were then bent so as to leave 

 the ring at right angles, and form a " stem " about 10 cms. in 

 length. A portion of the same iron specimen resembling 

 this stem both in form and insulation, but only leading just 

 up to the ring and back (see fig. 6), was bound on beside the 

 stem that compensation for its resistance might afterwards be 

 made. The ends of the platinum thermometer-wire, together 

 with a similar compensating resistance formed of a piece of 

 the same platinum wire, were also led up the stem, which was 

 in all parts most carefully insulated with mica. 



The next step was to overwind the ring with three layers of 

 platinum wire to serve respectively as magnetizing coil or 

 primary, as heating coil, and as secondary. These wires, 

 each of which, like the thermometer- wire, was doubly-insu- 

 lated with asbestos in the way previously described, were 

 respectively 110, 150, and 100 cm. in length, and '6, *45, and 



