On the Electrical Resistances of Metallic Wires. 433 



As may be seen, the curves A and B for both plates approach 

 one another very nearly. The greatest difference between 

 two corresponding ordinates is about 1 per cent., or not more 

 than the differences which have occurred in experimenting 

 with the slice in one position. 



From this the inference may be drawn that a unilateral 

 conductivity for heat is present in tourmaline either to a very 

 small degree or, what seems more probable, not at all. 



Phys. Inst. Univ. Strassburg, 

 May 1884. 



XLVIII. On the Alterations in the Electrical Resistances of 

 Metallic Wires, produced by Coiling and Uncoiling. By 

 James Hopps, Superintendent of Workshops, Royal Indian 

 Engineering College, Cooper's Hill* . 



[Plate IX.] 



DURING the past winter, while assisting a pupil to make 

 some resistance-coils, I was struck with the great dif- 

 ference that often existed between the resistance of the wire 

 before it was coiled on to its bobbin and after ; the difference 

 was not always constant or proportional to the length of the 

 wires used, and appeared to be largely influenced by the 

 tension with which the wires were coiled on to the bobbins. 



Some of the coils altered so much after winding as to lead 

 to the supposition of their being short-circuited; and on un- 

 coiling several of these and recoiling at a lower tension, the 

 resistance would alter considerably. 



The diameter of the bobbin used also appeared to have great 

 influence in producing changes in the resistance. 



A great portion of the alterations appeared to be temporary. 

 As has been observed above, the differences were not always 

 constant ; neither were they regular in their signs, i. e. they 

 would sometimes show an increase and sometimes a decrease 

 of resistance, but generally the change would be one of increase. 

 Having resolved to make some efforts to clear up this matter, 

 I first sought to find what had been done by others in this 

 direction. A very careful search through books and also of 

 the Transactions of the Royal Society afforded no evidence 

 that this field had been even entered upon, i. e. any attempt 

 by experiment or otherwise to determine and account for the 

 effects of coiling and uncoiling on the electrical resistance of 

 metallic wires. 



After experimenting some weeks, I purposely attended the 

 meeting of this Society on February 23rd to hear a paper, 



* Communicated by the Physical Society. 



