ON THE B.A. UNITS OF ELECTRICAL RESISTANCE. 



15 



how this degree of accuracy was attained. Some such statement was per- 

 haps necessarj', considering the difficulty of controlling the temperature of an 

 inaccessible wire, even within 1° Centigrade. 



Arranr/ement ^'c. of apparatus. — The instruments used in these experiments 

 for resistance measurements were theWheatstone's bridge and Thomson's gal- 

 vanometer belonging to the Association. The arrangements in the low-tempe- 

 rature experiments were as in the annexed figure. At one corner of a large 



table is the bridge A B (see B.A. Report, 1864, p. 353*) ; by means of mercury- 

 cups at D and Gr, are inserted the flat coil and the coil being compared with it ; 

 at E and JF are similarly inserted the middle coils, which were always two of 

 the units, as small and as nearly equal in temperature-variation as possible. 

 X, Y, Z are three earthenware jars in which the coils are placed ; these stand in 

 a trough, V W, provided with a waste-pipe going to the sink. The jars were 

 kept constantly overflowing by means of a feed-pipe fitted with an ofi'set for 

 each. The temperature in all three jars was carefully observed, and it was 

 found that after the tap had been turned on for fifteen minutes or so the 

 temperature in all three in general became constant, and remained so within 

 a tenth of a degree for a long time. Now and then irregularities occurred, 

 which caused the rejection of the results concerned. Thin wires go from E 

 and from the contact-block C to the galvanometer at the other end of the 

 table. The last adjustments of the balance were made by observing the 

 spot on the galvanometer-scale with the telescope from where the ob- 

 server sits. The battery- circuit terminates at H and J, and is made and 

 broken by means of a treadle worked by the observer's foot. A small 

 Leclanche's cell was found sufficient to indicate a deviation from balance 

 of a tenth of a millimetre on the bridge-scale. Since the contact of the 

 block-piece could not be relied on within less than this, no higher battery- 

 power was ever used. 



Thermoelectric disturbances. — To avoid thermoelectric currents, oAving to 

 the junction of copper with brass at the block, the button of the block-piece 

 was never touched by the fingers, but always by means of two pieces of 

 wood, which were exchanged now and again to prevent heating. It was 



» Reprint, p. 119. 



