1881.] Determination of the Ohm in Absolute Measure. 113 



auxiliary magnetometer was of very simple construction, and was 

 read with a telescope and a millimetre scale, the distance between 

 mirror and scale (about metres) being adjusted to approximate 

 equality with that used for the principal magnet, so that disturbances 

 were eliminated by simple comparisons of the scale readings. Daring 

 a magnetic storm it was very interesting to watch the simultaneous 

 movements of the magnets. 



In the month of September the apparatus was remounted under the 

 direction of Professor Stuart, to whose advice we have often been 

 indebted. In order to examine whether any errors were caused by the 

 circulation of currents in the frame, as has been suggested by more 

 than one critic, insulating pieces were inserted, mercury cups at the 

 same time being provided, so that the contacts could be restored at 

 pleasure. But the principal changes related to the manner of suspend- 

 ing the fibre and supporting the box and tube. In order to eliminate 

 tremor, as far as possible, these parts were supported by a massive 

 wooden stand, resting on the floor and overhanging, but without 

 contact, the top of the metal frame of the coil. The upper end of the 

 fibre was fastened to a rod sliding in a metal cap, which formed the 

 upper extremity of a 2-inch glass tube. Near the other end this tube 

 was attached to a triangular piece of brass, resting on three screws, 

 by which the whole could be raised or lowered bodily and levelled. 

 Rigidly attached to this tube, and forming a continuation of it, a 

 second glass tube, narrow enough to pass freely through the hollow 

 axle of the coil, protected the fibre as far as the box in which the 

 mirror and magnet were hung. This box was. cylindrical and, about 

 3 inches in diameter. The top fitted stiffly to the lower end of the 

 narrow glass tube, and the body of the box could be unscrewed, so as 

 to give access to the interior. The window necessary for observation 

 of the mirror was made of a piece of worked glass, and was fitted air- 

 tight. 



On my return to Cambridge in October the apparatus was tested, 

 but without the full success that had been hoped for. At high speeds 

 there was still unsteadiness enough to preclude the use of these speeds 

 for measurement. Since it is impossible to suppose that the tremor is 

 propagated with sufficient intensity through the floor and massive 

 brickwork on which the coil is supported, the cause must be looked for 

 in the fanning action of the revolving coil, aggravated no doubt by the 

 somewhat pendulous character of the box, and perhaps by the nearness 

 of the approach between the coil and its frame at three points of the 

 revolution. 



At this time the experiment was in danger of languishing, as other 

 occupations prevented Mr. Darwin from taking any further part ; but 

 on Dr. Schuster's return to Cambridge he offered his valuable assist- 

 ance. Encouraged by Sir W. Thomson, we determined to proceed with 



