152 
DE. A. E. H. TUTTOX f)X THE ELASMOMETEE, 
little firmly screwed plate, from which springs a light arm pointing towards the recess 
between the blocks and suitably curved, carrying a little tabular termination, 
2 centims. by 1 centim. {p in fig. 3). Above this is a second similar table, p', the pair 
being separated by four little spiral sj^rings arranged at the corners, each fixed at its 
ends to both plates. 
Manipulation of the top milled head thus adjusts the two tabular supports as 
regards their distance a^^art; the lower one enables the observer to move them into 
the hollow between the blocks until they come right under the platinum-iridium 
knife-edges, and out again; and the milled head down below, and in a vertical line 
with the front edge of the basal slab, enables them to be raised or lowered at will, in 
order to press the plate of the substance which they support up against the knife- 
edges, or to remove it from that position. 
The two little tables p>'p', which may be considered as mechanical finger tips, are 
all that is necessary for the temporary support of the plate during the preliminary 
adjustments. But a slight addition is made to each of them to enable them, if 
desired, to be retenable during the whole determination. The addition consists of a 
similar table of gun-metal q, carrying a raised knife-edge along the inner side. It is 
fitted on the permanent table p' in a readily detachable manner, by means of a 
couple of flat oval-headed screws carried by the table p' near its ends, well out of the 
way of the plate, corresponding with two similarly shaped, holes in q ; the two fittings 
are rigidly attached together when the screws are rotated 90° The knife-edge side 
is arranged along the inner side of the table, so that for any given separation of the 
two knife-edges there is the maximum amount of clear space between the two 
“ finger tips” for the pressure-point end of the balance beam to operate in. 
The “ finger tips” are to be so adjusted that the gun-metal knife-edges lie precisely 
under the platinum-iridium edges, and they are then to be raised until the plate laid 
on the gun-metal edges is gently pressed up into full contact with the platinum- 
iridium edges. As the gun-metal edges are exactly under the others, no bending of 
tlie plate is possible. 
If the method of carrying out the determination of the bending of the plate from 
the initial position of true planeness is adopted, and the gun-metal knife-edges are 
consequently retained in action, they should only be allowed to exert the minimum 
pressure. This is easily found, for the instant it begins to be further reduced sudden 
total derangement of the interference bands occurs. A careful partial turn of the 
milled head in the reverse direction will then usually sutfice to bring the bands again 
into their former adjusted position, corresponding with the attainment of just full 
contact of the plate with the platinum-iridium knife-edges. 
For the more convenient setting of the long edges of the plate perpendicular to the 
platinum-iridium knife-edges, a pair of horizontal milled-headed adjusting screws are 
added to the “ fingers,” one to each ; they are mounted parallel to each other at the 
level of the j^late in small uprights from the sliders, which curve inwards towards the 
