A NEW INTERFERENTIAL FORM OF ELASTICITY APPARATUS. 
157 
The Control Ai^paratus. 
About the middle of the lower bar of the beam on the left side a little saddle is 
fitted, on the top of which is fixed a platinum-iridium cone, the “ control-point,” 
similar to the agate “pressure-point.” Control of the movement of this point, and 
therefore of the whole beam with its pressure-point, is afforded by a horizontal agate 
plate, mounted above the control-point in a fitting capable of extremely fine vertical 
movement. This fitting is a rectangular double bracket, which envelopes the saddle 
on three sides, above, behind, and belovr, the vertical back limb being long enough to 
leave about 4 millims. space for the bar of the beam to swing in. The agate plate is 
fitted underneath the top limb. The bracket is carried as part of tlie same casting at 
the head of a vertical hollow cylinder of gun-metal 2 '8 centims. long, fitted at its 
lower end with a steel split-collar furnished with a tightening screw, which enables it 
to be fixed at a convenient height to a vertical shaft, on the upper end of wTich it 
slides as far as the bore permits, rotation being prevented by groove and pin. Below 
the part down which the cylinder can slide the shaft thickens to 1'2 centims., and the 
whole of the lower portion slides tightly in a gun-metal column 2’8 centims. diameter 
and 12 centims. lono;, which is mounted below^ on a stout bracket fixed to the left 
side of the rectangular basal box on 'which the balance is mounted. An exceedingly 
fine screw" thread, w"hose pitch is one-fourth of a millimetre, has been cut w’ith the 
greatest possible accuracy on a part of the shaft w"itlnn the column ; rotation of the 
shaft is prevented by groove and pin, but it can be propelled upw^ards or dowmw^ards 
by a rotatable but vertically immovable nut, furnished wdth a correspondingly fine 
thread wuthin and w’-orm-wdieel teeth w'ithout. The column is broken to admit the 
nut at a point 2‘2 centims. from tlie top, and rigidity of the tw"o })arts is secured, as 
w^ell as a suitable sujiport for the bearing of a driving endless screw", by means ol a 
hollow- rectangular box-piece, open in front and behind, east with the two parts of the 
column. The diameter of the nut is somewdiat greater tlian that of the column, 
and the peripheral oblique teeth are 72 in number; one revolution of the endless 
screw rotates it to the extent of one tooth. This corresponds to the vertical movement 
of the shaft through 0'0035 millim. The bearino-s of the endless screw" are attached 
o o 
to the box by an enveloping double claw", cast as one piece ; the front beaiing at the 
crossing is a horizontal cylinder from w’hich the screw- shaft emerges. The end ol the 
latter is notched diametrically, to admit of the attachment of either a simple milled 
head 3'2 centims. diameter, whose short cylindrical stem bears inside its bore a 
transverse bar fitting the notch, or of a Hooke’s-joint arrangement, wfiiich brings the 
manipulating head nearer to the observer wuthout interfering w-ith the movement of 
the w'hole balance, and also enables a more delicate mode of rotation than a simple 
milled head to be employed. It consists of a short tube w-lth similar inner cross bar, 
carried at one end of an equally short but thick rod wliose other end hears one 
semicircular fork of the Hooke’s-joint; the <»ther fork forms one end of a gun-metal 
sliaft centims. long. The tA^'o semicirculai' forks are rotatable about a- ])air of 
