Mr. C. W. Siemens on a Resistance-measurer. 



271 



The conditions upon which such an instrument could be suc- 

 cessful appeared to be the following : — 



1. The employment of a zero method, by which the galvano- 

 meter-needle would always be brought to the direction of the 

 magnetic meridian or the same given point upon the scale and, 

 therefore, be independent of the unknown function of the angle 

 of deflection. 



2. The readings to be made upon a simple lineal measure di- 

 vided into equal parts signifying equal units of resistance. 



3. The employment of a single and unalterable comparison- 

 resistance. 



The apparatus constructed to fulfil these conditions is repre- 

 sented by the following diagram. 



Two equal and parallel helices, h and h' , are fixed upon the 

 common slide ss' } which moves in the direction of its length 

 between guide rollers. This motion is effected by the end s' 

 armed by a facing of agate, which presses against the face of 

 the metal curve c c r . The latter is fixed upon a slide moving in 

 a groove in the rule dd', at right angles to the direction of ss'. 

 The curve is moved in the direction dd', by means of a milled 

 head i, on the axis of which is a pinion gearing into a rack un- 

 derneath the straight edge of the curve c c'. The rule dd 1 is gra- 

 duated into equal parts ; and opposite to the divisions is a nonius 

 up the straight edge and the curve, to divide each degree into 

 ten parts. Whenever the milled head i, therefore, is turned, the 

 position of the curve is altered ; and as the point s' of the bob- 



