ON A QUARTZ THREAD GRAVITY BALANCE. 231 



repeated six times. From the comparisons a table was prepared showing the correc- 

 tions to be applied at each point of the scale. The arrangement of coils adopted 

 allows us to measure each temperature in two ways, and at every five degrees in 

 three ways ; this provides a valuable check against large errors or mistakes. 



It may possibly be considered that as we have to refer to a table of corrections in 

 any case, we wasted time in adjusting the coils with the nicety we employed. This 

 is not the case, however, for in the process of observing it is necessary to make many 

 approximate estimates of temperature as well as the final maximum, and the unconnected 

 direct readings are very useful for this purpose. We must acknowledge the assistance 

 we received from a piper by Mr. GRIFFITHS in ' Nature,'* in which the construction 

 of a box of coils and a standard thermometer is described. 



We usually observe by closing the galvanometer circuit before we close the battery 

 circuit. A special experiment showed that this was legitimate with our inductionless 

 coils and thermometer, and thus we are not troubled by residual thermoelectric 

 effects ; rendered small in any case by the design of the contact maker. 



The Arrester. 



The arrangement made for arresting the lever is in reality more simple than the 

 description would seem to indicate. We have a framework adjustable to the bars of 

 the main supports by means of clamp screws. This framework carries two jaws 

 worked up and down by means of a pinion with teeth cut by an involute cutter. The 

 rack teeth are therefore triangular, the tops being cut off; it is necessary to have a 

 good smooth motion. The lower jaw of the arrester is convex so that the lever is 

 held against a convex surface. The upper jaw is wedge-shaped, and presses the lever 

 against the convex surface of the lower jaw. This arrangement was adopted with a 

 view to preventing the lever getting bent by the arrester. The active surfaces of the 

 jaws are connected to the racks through the intermediary of spiral springs, and the 

 upper jaw has a stronger spring than the lower jaw. By a simple mechanical 

 arrangement the lever is always held by the same pressural force, and this depends on 

 the elasticity of the springs, and not on the exact angle through which the arrester 

 shaft may have been turned. The arrester frame is adjusted until the lever is 

 practically arrested in its observing position and is not displaced out of the field of 

 view of the microscope. What happens is this. As the jaws close the upper jaw 

 reaches a fixed position. On further turning the arrester handle the spring of the 

 jaw gets compressed but the jaw remains fixed. Then the lower jaw closes on the 

 upper one, and on further turning its spring gets compressed. An essential part of 

 tliis construction is that the spring of the lower jaw must be weaker than that of the 

 upper one. The arrester shaft of course works through a Hook's joint. Outside the 

 thermometer end of the machine a milled head is placed, and this works a key which 



* November 14, 1895. 



