114 Dr. J. A. Fleming on a neio 



for, as Prof. Chrystal has remarked in his report (Brit. Assoc. 

 Report, 1876), it is not easy to tell whether the temperature 

 of the water in which the coil rests is identically the same as 

 that of the ware, since the latter is imbedded in a mass of 

 slowly conducting paraffin. To reduce as far as possible the 

 difficulty of keeping the coils at a constant temperature, they 

 are placed in water-vessels made of zinc (see fig. 3, PI. IV.). 

 These water-boxes are composed of two cylindrical vessels — an 

 outer case 9 inches high and 8 inches in diameter, and an inner 

 one of lesser size ; the two are connected at the top, so that they 

 form a sort of jar with hollow sides and double bottom. This 

 interspace forms an air-jacket. Around the inside vessel near 

 the top is a row of small holes; and two tubes communicate at 

 the bottom — one with the inner vessel, and the other with the 

 annular interspace. The top is closed by a wooden lid with 

 apertures for thermometer and stirrer. Water can be made 

 to flow from the supply-pipes into the inner vessel ; it rises up 

 and overflows through the holes, and drains away down the 

 interspace and out by the other pipe. The bodies of the four 

 coils are placed in four w T ater-boxes of this description ; and 

 water from the town mains being sent in a continuous stream 

 through all four water-boxes, the coils are rapidly brought to 

 and maintained at a known temperature. Any desired tem- 

 perature can be given to one coil by leading warm water from 

 a cistern into its vessel. The annular air-filled space renders 

 the rate of cooling very slow. Hence the coils, once at the 

 desired temperature, can easily be kept there. Fig. 4, PI. IV., 

 gives a sketch of the arrangement, two of the water-boxes 

 being removed to show the connexions. 



The advantage of the somewhat complicated arrangement of 

 copper bars will now T be seen. We can, without withdrawing 

 the coils 3 and 4 from their water-boxes, and without in any 

 way disturbing the other arrangements, reverse the position of 

 the coils 3 and 4 on the bridge, by simply lifting up the legs 

 half an inch and changing the mercury-cups into which they 

 dip. Thus the legs of coil 3 are changed from cups a and c 

 to b and d, and those of coil 4 from V and d! to a' and c f . 

 This exchange does not occupy more than a few seconds; and 

 hence we can obtain the two readings necessary to give the 

 difference of the resistance of the coils 3 and 4 when they are 

 at different temperatures in a very short time. During this 

 short time the temperatures of the two coils will not change 

 perceptibly, protected as they are by an air-jacket. 



In the ordinary form of straight bridge there is considerable 

 trouble in exchanging the coils, because the water- vessels have 

 to be moved and the mercury-cups readjusted ; and all this 



