

A NEW CURRENT WEIGHER, ETC. 



471 



The position of the balance beam is observed by viewing a finely divided scale 

 carried by the pointer through a microscope seen in fig. 2, Plate 7, and in fig. 4. 



A double glazed case or cover, with f-inch air space between the sheets, resting 

 on a phosphor-bronze plate, serves to exclude dust and draughts, and to minimise 

 convection currents which may be caused by unequal radiation or conduction from 

 surrounding objects. 



The whole instrument is supported on an adjustable phosphor-bronze stand or 

 pedestal at a convenient height (see fig. 4), levelling screws being placed at the 

 corners of the base. 



SECTION 3. ADJUSTABLE SUPPORT FOB BALANCE. 



On opposite sides of the central pillar of the pedestal (see fig. 4) are sliding 

 brackets BB, like the tables of small milling machines, which can be lowered through 

 distances of about 14 inches (35 centims.) by means of vertical screws SS. Each 

 bracket supports a slide rest having a circular top-plate which can be moved 

 half-an-inch horizontally in two directions at right angles by means of screws with 

 graduated heads. The nuts on the vertical screws are of large diameter, and they 



Fig. 5. Section through top-plate of slide rest for supporting fixed cylinders. 



and the heads of the horizontal screws are divided to read thousandths of an inch. 

 As each division can be subdivided by eye to tenths, it is possible to read the position 

 of either fixed cylinder to a ten-thousandth of an inch. 



The weights of the fixed cylinders and brackets are sufficient to overcome the 

 friction in the vertical slides and thus avoid backlash in these motions. In the 

 horizontal movements backlash is avoided by using strong phosphor-bronze springs 

 shown at a, fig. 5, capable of moving the corresponding slide when tightened up 



