124 THE SCIENTIFIC PAPERS OF 



upon a magnetic point, and from the given distance of the coils 

 from each other. I prefer, however, in practice to determine the 

 curve of each separate apparatus empirically, because it is not pos- 

 sible to coil a helix mathematically true, or to set it, when coiled, 

 absolutely at right angles to the plane of its horizontal motion. 



In the determination of each curve I use a delicately adjusted 

 rheostat or scale of resistances in the circuit of -x, giving it 

 varying values corresponding to the equal divisions of the en- 

 graved scale, and constructing the curve according to the position 

 which it is found necessary to give to the point s' in order to 

 arrive at the magnetic balance. With each instrument it would 

 be possible to have two values of R one expressed in mercury 

 and the other in B.A. units ; and in order to measure at pleasure 

 in either of these units, it would only be necessary to insert the 

 one or other between the terminal screws for R. 



The instrument has been found to be very convenient for the 

 measurement of the wire resistances of overland lines, or for the 

 reading of resistance thermometers ; it reduces the operation to 

 the observation of the zero position of a needle, and the reading 

 upon a graduated scale, which can be performed by a person of 

 ordinary intelligence without experience in electrical measurement. 

 In accuracy and range it fully equals the bridge method, while 

 as regards portability and cheapness of apparatus the advantages 

 are decidedly in its favour. 



In tlw discussion of his Paper 



"ON PYROMETERS," 



MR. SIEMENS * said : With regard to the first question whether 

 any permanent change occurs in the conductivity of the metal wire 

 when exposed to heat, I have found that such is not the case. 

 The wire no doubt elongates when exposed, but with it the lateral 

 dimensions increase. In taking the resistance of the wire, we do 

 not deal with the length only, but with the length divided by the 



* Excerpt Journal of the Iron and Steel Institute, Vol. I. 1869-70, pp. 54, 55. 



