138 EEPORT— 1893. 



the current traversing it. If this adjustment be made we will say that 

 the apparatus is in an equilibrium position.' ' 



If M=coefEcient of mutual induction of standard coil and circum- 

 ference of disc, 

 w=rate of rotation of disc (number of revolutions per second), 

 R=resistance, 

 y=current through standard coil and resistance, 



then in an equilibrium position 



M??.7=Ry, 

 or E=Mh. 



I do not think that electricians have as yet realised the accuracy and 

 ease with which absolute measurements of resistance may be made by 

 this method. The absolute measurement involves measuring first the 

 coefficient of mutual induction of the standard coil and the circumference 

 of the rotating disc, and secondly the rate of rotation of the disc. 



Now it lies well within the resources of modern mechanical engineer- 

 ing to make a standard coil and disc of dimensions known to an accuracy 

 considerably greater than 1 in 10,000, the coil being constructed of a 

 single layer of wire wound in a screw thread cut on a cylinder of large 

 diameter ; and the measurement of the rate of rotation to equal accuracy 

 is a simple matter. There is difiiculty in maintaining a rate of rotation 

 constant to this figure for four or five minutes, but with the closest 

 attention to the lubrication of all the bearings this also might be accom- 

 plished. Such constancy is well worth striving for, as the ease with 

 which measurements of resistance can be made by the method largely 

 dependu upon it. 



I do not propose on this occasion to enter into the details of the 

 method I have adopted in making the measurements, the results of which 

 I have now to bring before the Section. But it will perhaps be of 

 interest if I say a few words about the time-measurement. 



In measuring a resistance we have to find the rate of rotation corre- 

 sponding to an equilibrium position. It is easiest in practice to determine 

 this by interpolation from two determined rates of rotation (near together, 

 and respectively slower and faster than the required rate) and the 

 galvanometer deflections corresponding to them, so that each determina- 

 tion of resistance involves two determinations of galvanometer deflection 

 and the rates of rotation corresponding to them. 



In order that the galvanometer deflection may be obtained with 

 sufiicient accuracy from a limited number of reversals (in my observations 

 the number has been almost uniformly thirty-three, taking about four 

 minutes in each case) the brush at the circumference of the disc needs 

 to be perforated and to be supplied with a constant stream of mercury. 

 Such a brush in its best condition almost entirely eliminates the continual 

 jerking of the galvanometer needle consequent on thermo-electric changes 

 at the point of contact of brush and disc. A multiplication of such 

 brushes at three or four points of the circumference would do this even 

 more completely. 



During the four or five minutes' run the rate of rotation is referred by 



' Vide Phil. Trans., 1891, A, p. 2, ' On the Determmation of the Specific Eesistance 

 of Mercury in Absolute Measure.' 



