302 E. B. Rosa — Determination of v, the Ratio of the 



scale division, corresponding to less than asVoth part of the 

 whole current. To obtain a regularity of action which per- 

 mitted such accurate observations required a very delicate 

 adjustment of the distance between the surfaces of the mercury 

 and the points above, as well as clean surfaces and a steady 

 current. 



5. Battery. — About forty cells of a storage battery with a 

 total electromotive force of about eighty volts were used. A 

 higher electromotive force, at first proposed, was thought to 

 be unnecessary. 



6 Resistances. — The resistance a was taken from a box of 

 Elliott Bros., whose total resistance was about 12,000 ohms ; 

 the resistance d was a 100,000 box from the same firm. The 

 first of these, box A, was carefully calibrated by comparing the 

 several coils on a Fleming bridge with three standard coils of 

 10, 100 and 1000 B. A. units respectively. The first was a 

 Warden Muirhead No. 292, 10 B. A. Units. Its value, deter- 

 mined by Glazebrook, Oct., 1887, is 9-99116 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 care- 

 fully compared with the known resistances of A. The tem- 

 perature-coefficients of both boxes were also carefully deter- 

 mined. 



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 ap- 

 pears as 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 

 was prepared and mounted, connection 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 re- 

 sistances 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 resistances proved 

 quite constant and reliable. Two were used in this experiment, 

 R a and R 6 , whose resistances were approximately 1,570,000 and 

 2,440,000 ohms. During the six weeks preceding May 9 their 

 alteration, aside from temperature fluctuations, was inapprecia- 

 ble. 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 



