480 PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



outer surfaces were thus practically concentric, and the ends at right angles to the 

 axis. The turning was necessarily slow, more than five weeks being occupied on each 

 of the large cylinders ; the winding of the coils was, however, completed in a few 

 hours. Alternate cuttings were made of the spirals, of which the grooves were 

 V-shaped, with an angle of 85, and of -j- 8 - inch pitch. It was very important that each 

 groove should be midway between its neighbours, and the lathe was operated to effect 

 this ; subsequent microscopic examination proved the equality of distance. While in 

 the lathe, the diametral uniformity of the grooves was tested by winding in different 

 parts of the cylinder a couple of turns of No. 24 copper wire, and estimating the 

 difference in the diameters of the various turns by the touch of callipers. The 

 cutting tool used was hardened in mercury and was not tempered. 



On each large cylinder there are two pairs of coils, the central portion being left 

 unwound for an axial distance of -fa of an inch. The leads of all the coils must lie in a 

 plane containing the axis of the cylinder, or otherwise the current through them will 

 exert a force on the current in the suspended coils of the balance. To ensure the 

 absence of such a force, the following scheme was adopted for the winding of each coil 

 (see fig. 8, p. 473). From the outside of the cylinder and near one end, two radial cylin- 

 drical holes, a and b, were drilled ; these are - inch in diameter, - 3 - 2 - inch from centre 

 to centre, and 1^ inches in depth ; they lie in a plane containing the axis of the coil. 

 From the inside surface two other - 6 - 4 -inch holes, c and d, were drilled to a depth of 

 l inches in the same axial plane as the others ; one of these, d, is near the centre of the 

 cylinder, and the other, c, is - 3 A 2 - inch from b, the innermost of the previous ones. The 

 holes a and b have slotted brass nipples, shown in section, screwed into them, and c 

 and d are bushed on the inside of the cylinder with ivory pieces. After these bush 

 pieces were screwed into position, the fine radial holes passing through the nipples, the 

 ivory, and the marble, were drilled ; the diameter of these holes is 0'024 inch, and they 

 admit of the free motion of a straight piece of No. 24 wire. The radial holes were 

 drilled in the following manner : A bar of steel, 2 inches square and 30 inches long, 

 with two opposite planed surfaces, was bolted to the slide rest so that it projected 

 towards and was perpendicular to the axis of the lathe ; a ^-inch hole was then 

 drilled through the far end of it. Into this hole a spindle was fitted, and on the 

 spindle a small pulley was fixed, so that the whole could be driven by a motor. The 

 bar was then turned into an axial position, the spindle set parallel to the face plate, 

 and the radial holes drilled by a fine drill fitted in the spindle head ; the feed was 

 governed by the pressure of the hand. A check on the accuracy with which the 

 holes were drilled was obtained in the following manner : Adjacent helices are 

 supposed to start in the same diametral plane and at an angular distance of 180 

 apart ; the prolongations of the holes a, 6, c, d should therefore be in line with those 

 drilled for the leads of the adjacent coil. To test this, a straight piece of No. 24 

 wire was passed through corresponding holes, and pulled taut ; there was no undue 

 friction, and a centre finder indicated that the wire cut the axis of the cylinder. As 



