58 



REPORT — 1888. 



for the coil Y. For the standard coil Flat this carve is accarately a 

 straight line. This coil was then kept at 0°, and the temperature of X 

 varied, and so on for all the coils. 



Now, at the temperatures given in Table I., column 4, taken from 

 the 1867 Report the resistances of all the coils should be the same. 

 Fleming found that this was not quite strictly true. He defines therefore 

 as the Mean B.A. Unit the mean of the values of the coils at the temper- 

 atures at which they were originally said to be equal. This value is 

 shown on his diagram by a red horizontal line. 



For the coils of platinum silver alloy, which is now used for standards, 

 Fleming's results are accurately represented by straight lines for the tem- 

 perature curves. This, however, is not so strictly the case for the coils 

 A and B of platinum iridium "alloy; thus for these two coils Fleming 

 took observations in the neighbourhood of 0°, 4°, 8°, 15°, and 21°, 

 numerous observations being made at each temperature ; the straight line 

 on the chart joining the means of the observations at 15° and 21° passes 

 considerably above the observations at 0°, 4°, and 8°. The same too is 

 the case, though in a less marked degree, for the platinum coils D and B. 

 In the chart as drawn by Fleming it has been assumed that the tempera- 

 ture curves are straight lines, and these have been drawn to represent all 

 the observations as closely as possible, but the differences are considerable. 



If we draw curves instead of straight lines to represent Fleming's 

 experiments these curves between 10° and 20° are in all cases nearly 

 straight, and the differences, at the two temperatures, from Flat at 0° are 

 given in bridge wire divisions in Table II. 



Table II. 



We could determine from this table the temperatures at which the 

 various coils are equal, and hence compare Fleming's results with those 

 of previous observers ; it will be easier to do this after discussing our 

 own observations. 



During the past year and a half the coils have again been examined- 

 by ourselves. We find that between the temperatures of about 10° and 

 20° Centigrade the resistances of the coils, including an eighth coil H 

 (No. 6 of the Report of 1867), may be represented by the formulae given, 

 in Table III. 



In obtaining the table it has been assumed, in accordance with the 

 observations of Dr. Fleming, confirmed by Lord Rayleigh and ourselves, 

 that the resistance of one division (about 1 mm.) of our bridge wire at a 

 temperature of 15° is -0000498 B.A.U. The table gives in B.A. Units 

 the value of R,— Flat^, R^ being the resistance of the coils in order at 

 temperature t°, F]at„ that of Flat at 0°. 



