and Electrical Resistance of Iron. 219 



the first. The other two arms contained equal resistances ; 

 the 200-ohm coil of a resistance-box in the one ; and in the 



Fig. 1. — Form of Bridge for Temperature Measurements. 



other the smaller coils of the box (together making 200 ohms) 

 with just so many tenths of an ohm short-circuited as were 

 in the remainder CB of the slide-wire. It is clear then 

 that, if C is the position of balance, the resistance AC is 

 equal to the nett resistance of the thermometer-wire. 



The resistance of the slide-wire, whose calibration curve 

 was very uniform, was about 1*4 ohms. This was approxi- 

 mately that of the thermometer-wire at 1200°, so that the 

 balancing point for all lower temperatures was within the 

 range of the metre-long slide-wire. The resistance per unit 

 length of the latter was conveniently obtained by inserting a 

 standard ohm in the place of the thermometer-wire. 



The above method was also applied to the measurement of 

 the resistance of the iron in the core. 



Insulation. — The chief difficulty of the method of these 

 experiments lies in so insulating the wires themselves and 

 the various layers of the ring-magnet from one another, that 

 the insulation shall still remain good at the highest 

 temperatures reached. 



The only material which is practically applicable to the 

 insulation of the wires is asbestos paper ; this paper, which 

 though fairly thin is by no means uniform, was used in the 

 form of long narrow strips wound spirally over the wires to 

 be insulated. After considerable practice, it was found 

 possible to cover wires even as small as *3 mm. in diameter 

 in this way with a reliable, and fairly thin and uniform 

 covering ; for such wires, a strip about 4 mm. wide was 

 found to be most suitable. 



Asbestos paper must of necessity contain starch or some- 

 thing similar to hold it together ; and experiments proved 

 that, when heated in a non-oxidizing atmosphere, the resulting 

 carbon deposit reduced the insulating power of the asbestos 

 to a very low value. This difficulty was -overcome by 



