480 



REPORT — 1867. 



The condition ii^Don which such an instrument could bo successful appeared 

 to be the following : — 



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

 should always be brought to the direction of the magnetic meridian, or the 

 same given point upon the scale, and therefore bo independent of the unknown 

 function of the angle of deflection. 



2. The readings to be made upon a simple lineal measure divided into equal 

 parts signifying equal units of resistance. 



3. The emplo3-ment of a single and unalterable comparison-resistance. 

 The apparatus constructed to fulfil these conditions is represented by the 



following diagram : — 



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

 slide s s , 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 d d' , at right angles in the direction d d' by 

 means of a milled head /, on the axis of which is a pinion gearing into a rack 

 underneath the straight edge of the curve c c. The rule d d' is graduated in 

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

 lows it in all its movements. 



The wires of the two bobbins are connected together, in the common point 

 a, with the pole of a galvanic battery e, the other pole being connected with 

 two resistances E, and through these with the remaining end of the galvano- 

 meter-helices. The resistance E is made constant, and adjusted so that when 

 _^,_0 the index of the curve stands exactly opposite the zero of the graduated 

 scale d d, the unknown resistance being represented by .r. 



It is evident that, the resistance in the bobbins being equal, as also their 

 dimensions and initinl magnetic efi^ects upon the needle suspended between 

 them, if we make the resistance x equal to E, the current in the two branches 



