122 THE SCIENTIFIC PAPERS OF 



method a well adjusted variable-resistance coil is necessary, which, 

 if the method is intended to be applicable between wide limits, 

 will have impracticably large dimensions. The bridge method, 

 though very beautiful, requires three adjusted coils, and frequently 

 gives rise to calculation, which renders it unavailable for unskilled 

 operators. The sine method, which is the most suitable for 

 measuring great resistances, requires even a superior amount of 

 skill and mathematical knowledge on the part of the operator. 

 Many years' experience of these methods made me feel the want of 

 an instrument which would, by its simplicity of construction and ease 

 of manipulation, be capable of employment by an unskilled operator 

 with a degree of correctness equal to that of the bridge method. 



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 galvanometer- 

 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 qf deflection. 



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

 divided 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 diagram, Plate 11. 



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'. 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 under- 

 neath the straight edge of the curve c c'. The rule d d' 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 , therefore, is turned, the 

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

 bin slide is pressed against it by means of a spring, the bobbin 

 follows it in all its movements. 



