1881.] Determination of the Ohm in A bsolute Measure. 141 



at the temperature at which they are supposed to be correct, aud the 

 ohm as determined by the Committee is, of course, uncertain within 

 the limits within which the standards differ, but for our present pur- 

 pose these may be considered identical. The coils were carefully 

 compared by Mr. Fleming, who also determined their temperature 

 coefficient. One coil in a flat case (hence called the " flat coil "), which 

 had the smallest temperature coefficient, and supposed to be right at 

 14 0, 8 C, was taken at that temperature as the true ohm. Six of the 

 standards were so arranged and joined together by means of mercury 

 cups, that four were in a row, and the remaining two in double circuit, 

 the whole system of coils being thus equivalent to about 4 - 5 ohms. 

 Our standard German silver was nearly 4 - 6 ohms. As the dif- 

 ference was too great to allow a direct comparison by means of Mr. 

 Fleming's bridge, a piece of German silver wire was prepared so as to 

 have a resistance of '1 ohm ; this could easily be done within the required 

 limits of accuracy by means of a set of resistance coils belonging to 

 the Laboratory. Having thus a set of resistances very nearly equal 

 to the one to be measured, a series of experiments was made on two 

 successive days. Knowing all the temperature coefficients, we could 

 easily reduce the measurements to ohms. Four different experiments 

 gave for the German silver standard at 8° '5 C. — 



4-5902 



4-5896 



45869 



4-5879 



4-5890 



Mean=4-5887 



Assuming the German silver to vary 4*4 per cent, for 100° C, we 

 find for our standard at ll 0- 5 C. 4 - 595 ohms. We have hitherto 

 neglected to take account of the resistance of the two stout copper 

 rods which connected the rotating coil with the resistance bridge. 

 This resistance was determined by Mr. Fleming to be '003 ohm. To 

 make matters equal, we ought to have added that resistance to the 

 British Association standards in comparing them with the standard 

 used by us, and we should then have found that the absolute resistance 



found by us to be equal to 4* 543 earta quadran t^ g ^ ^ 4.50,2 



second 



ohms. 



From this we calculate that the ohm as fixed by the Committee of 

 the British Association is — 



•9893 



earth quadrant 



second 



this being the final result of our experiments. 



