1G2 
DR. A. E. H. TUTTON ON THE ELASMOMETEE, 
l)y taking the reading of the last whole millimetre from the scale on the guiding hed, 
and then adding the reading for the fraction of a millimetre obtained from the di’um; 
the first two places of decimals are given directly by the numbered circle next the 
hearing, and the third place is immediately indicated by the red number above the 
circle whose intersection with the oblique line falls exactly under the red indicating 
line. In practice this mode of determining the thousandth of a millimetre proves 
the easiest the author has yet worked with. The fact that a single screw is alone 
concerned, and one which is absolutely freed from backlash by the double-nut device, 
renders the method highly accurate. 
The microscope is required for the measurement of the free length of the plate of 
the substance under investigation, that is, the distance between tbe platinum-iridium 
knife-edires a<minst which it is bent: also for the measurement of the breadth ot the 
plate. For the determination of the plate’s thickness, the thickness measurer which 
the author has previously described (‘Phil. Trans.,’ A, 1898, vol. 191, p. 337) is 
admirably adapted ; it aftbrds the thickness with accuracy to the same unit as the 
measuring microscope, namely, the thousandth of a millimetre. 
The most convenient length of plate is from 10 to 20 millims., so as to afford a 
length lietween the knife-edges of not less than 8 and upwards of 16 millims. For 
a 15-millim. plate a breadth of about 3 millims. and a thickness of 0'4 to 0'7 millim. 
are suitable. 
In carrying out the determination of the distance separating the two platinum- 
iridium knife-edges several series of readings are obtained for the separation of the 
points A and C, and of B and D, and the mean taken. In case the alternative knife- 
edges are employed, the actual distance between the centres of the two edges can be 
read off directly, in addition to the separation of the end-points. 
The lu'eadth of the plate is determined for six or seven equidistant points along 
that portion of the jDlate which lies between the knife-edges, and the mean 
taken. 
It is convenient to engrave a little cross with a fine needle at the exact centre of 
the jDlate while under the microscope, so that the platinum-iridium point of the 
transmitter can suljsequently be readily adjusted precisely at tlie centre with the aid 
of a pocket lens. 
Instead of the method of carrying out the determinations from the plane of no 
liending of the plate, by use of the spring tables and their gun-metal knife-edges, it 
may be more generally preferred to employ the method used by Koch, of commencing 
with the plate slightly lient under a constant initial weight, say 50 to 100 grammes, 
and the apparatus lends itself admirably to this method. 
Details of the modes of carrying out the determinations, together with the 
description of a delicate mode of feeding the balance-pan with the weight employed, 
are reserved, however, for a subsequent communication describing the work actually 
carried out with the instrument. 
