110 Dr. J. A. Fleming on a new 



a modification of the usual form of Wheatstone's bridge would 

 render these processes more expeditious and at the same time 

 more accurate. It is the object of the present paper to de- 

 scribe a form of resistance-balance which has been recently 

 constructed for the Cavendish Laboratory, and which expe- 

 rience shows to have several decided advantages over the old 

 form. 



2. Description of the Resistance-balance. — A circular disk of 

 mahogany 18 inches in diameter and about 1 inch thick (/) 

 (Plate IV. figs. 1 and 2) stands upon three short feet L. Upon 

 this, and concentric with it, is screwed down a disk of ebonite 

 14 inches in diameter and j of an inch thick (e). This ebonite 

 disk has a semicircular groove turned in its circumference. The 

 circular wooden base extends on one side into a narrow rect- 

 angle (j) 4 inches wide and of the same thickness as the disk. To 

 this are connected two other rectangular pieces (h, i,) which are 

 joined together by slotted brass bars (y, see fig. 2) underneath, 

 in such a manner as to permit the two intervals to be made 

 wider or narrower at pleasure. This promontory is of wood, 

 of the same material and thickness as the disk /, and is sup- 

 ported and levelled by three levelling-screws n, n f ,n f/ . Through 

 the centre of the ebonite disk passes a brass centre-pin D D / 

 (fig. 2), on which is centred a brass arm, H B 7 , capable of 

 revolving round just clear of the disk. Beneath the arm, and 

 soldered to it, is a short brass spring x, which depends verti- 

 cal^ downwards. This spring carries at its extremity a small 

 prism of platinum-iridium with one edge vertical and turned 

 inwards. In the groove turned in the disk e is stretched a 

 platinum-iridium wire about -^ °f an mcn ni diameter. The 

 wire extends round about |-J of the circumference, and is 

 about 39 inches long ; and the groove is of such a size that 

 the wire lies with exactly half its thickness imbedded in it. 

 This wire is represented by the thick black line A C A' in 

 fig. 1. The ends of this wire are soldered to copper strips 

 k, k. On the wood rectangles j, h, i is fastened an arrange- 

 ment of longitudinal copper strips, k, k, which connect toge- 

 ther eight transverse square copper bars in the manner shown 

 in fig. 1. On the ends of these transverse bars are fixed ver- 

 tical copper pins ^ of an inch in diameter and j of an inch 

 high. On these pins are slipped short lengths of india-rubber 

 tube which extend beyond the pins, so that they form small 

 cups about 1 inch deep, p 1 (see fig. 3). The top of the copper 

 pin is well amalgamated with mercury, and forms the bottom 

 of the cup. These cups are filled about a quarter full of mer- 

 cury. On the longitudinal strips of copper are fixed three 

 binding-screws B, B' ; G; and a fourth (G 7 ) simply goes through 



