A NEW CURRENT WEIGHER, ETC. 499 



SECTION 7. ERECTING AND ADJUSTING THE INSTRUMENT. 



To facilitate the setting of the fixed cylinders on the balance table, two spirit 

 levels and four sets of cross-wires are mounted on the upper plane end of each. The 

 sensitiveness of the levels is such that a tilt of 20 seconds of arc displaces the air 

 bubbles 1 millim. from their central position. A Whitworth surface plate was 

 levelled and on this the spirit levels were set; afterwards a marble cylinder was 

 rested on the plate, which was then relevelled, and two other levels placed at right 

 angles on the upper end of the cylinder ; the displacement of a bubble from its mid- 

 position was practically unreadable, the parallelism of the plane ends of the cylinder 

 being thus confirmed. The levels were screwed to the cylinder and re-set to read 

 correctly. 



At opposite ends of two diameters at right angles, four slides with upright pieces 

 carrying cross-wires are screwed to the upper plane end of each cylinder (see fig. 2, 

 Plate 7). These are adjustable in azimuth and the final setting is such that the line 

 joining the points of intersection of opposite cross- wires lies in a plane containing the 

 axis of the coils. The setting was made by suspending a weighted thread inside the 

 cylinder so as to coincide with the axis, the indicator of thjs adjustment being a 

 centre finder. A cathetometer telescope was next focussed on the thread and on one 

 of the cross-wires, and was altered in position until the plane of the vertical wire of 

 the telescope lay in the same plane as the thread and the vertical cross-wire. The 

 cross- wires opposite to this latter were then adjusted in azimuth until they also lay in 

 this plane. 



Each suspended cylinder carries a brass T-piece supporting two sjjirit-levels at 

 right angles ; in addition a tripod is supported which in turn carries a pointed rod 

 to be seen projecting above the fixed cylinder in fig. 2 (Plate 7). The ends of the 

 tripod legs enter into the cylinder and are turned to be an exact fit. The rod is 

 for the adjustment to coincidence of the axes of the fixed and suspended coils ; 

 it is adjustable in vertical height and its pointed extremity lies in the axis of the 

 coils ; it is set so that when its extremity is in the plane of the cross-wire inter- 

 sections the suspended and fixed coils are symmetrical as regards vertical height. 

 The coils are concentric when the lines joining opposite cross-wires intersect in the 

 axis of the rod. 



Concentric cable is used for the leads to and from the various coils. The junctions 

 of the cable with the fixed coils are shown in fig. 8, and those with the suspended 

 coils in fig. 10. In the case of the fixed coils the ends of the wire leading to any one 

 of the coils were first soldered to small brass blocks supported by a strip of ebonite 

 which in turn was screwed to the cylinder ; the ends of the leads of the concentric 

 cable were similarly soldered to two small brass pieces which were screwed in contact 

 with those leading to a coil. The cable could thus be easily removed without in any 

 way damaging the connecting pieces. In the case of the suspended coils, the wires 



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