320 Mr. E. B. Rosa on the Determination o/v, the 



comparing the several coils on a Fleming bridge with three 

 standard coils of 10, 100, and 1000 ohms respectively. The 

 first was a Warden-Muirhead No. 292, 10 B. A. U. Its value, 

 determined by Glazebrook, Oct. 1887, is 9-99416 at 16°'5 C. 

 The other two had been previously carefully compared with 

 this. The values of the resistances of box A adopted were the 

 means of three different and closely agreeing determinations, 

 made at different temperatures. The several coils of box B 

 were carefully compared with the known resistances of A. 

 The temperature-coefficients of both boxes were also carefully 

 determined. 



The resistance c was of graphite. Plate-glass was ground 

 with fine emery and lines ruled upon it. Under a magnifying 

 power of several hundred diameters the layer of graphite 

 appears made up of patches which run together at numerous 

 points. The resistance of a strip of graphite of given length 

 and breadth depends upon how well these patches are joined 

 together. The glass and graphite are given a heavy coat of 

 shellac and thoroughly dried. A series of ten such resistances 

 were prepared and mounted, connexion being made at the 

 ends by tin-foil, held firmly in contact with the graphite by 

 rubber packing, wires passing out from the tin-foil. The 

 resistances were placed in cylindrical boxes with vulcanite 

 tops, in which were set binding-screws, joined to the wire 

 terminals. The boxes can be surrounded by water or other 

 material to lessen the temperature fluctuations. These re- 

 sistances proved quite constant and reliable. Two were used 

 in this experiment, R 2 and R 5 , whose resistances were ap- 

 proximately 1,570,000 and 2,450,000 ohms. During the six 

 weeks preceding May 9, their alteration, aside from tempera- 

 ture fluctuations, was inappreciable. But between May 9 and 

 May 13, when not in use, from some as yet unknown cause, 

 both increased about one half of one per cent., and up to June 

 8, when last used, remained nearly constant at the new value. 

 Inasmuch as glass and shellac are poor conductors, the tem- 

 perature of the graphite resistances cannot safely be assumed 

 to be the same as that of the air within the box, unless the 

 latter has been kept constant for some time. In order, there- 

 fore, to avoid all uncertainty as to their values these resist- 

 ances were determined anew whenever used; and if their 

 temperature changed materially, both just before and just 

 after using. They were compared with the resistances of 

 boxes A and B, two arms of a Wheatstone bridge, with a ratio 

 of 99*89, being taken from A. Here is a specimen observation 

 and calculation: — 



