y 
f 
220 ON THE ABSOLUTE MEASUREMENT OF HARDNESS. 
‘ 
pressureless contact, after the effect of each of the successive step loads 
had been brought to bear, and thus the load corresponding to the oceur- 
rence if an indentation in p was indicated with certainty (see below). 
V. GENERAL REMARKS ON THE OBSERVATIONS. 
Apart from special methods of research the general plan of the ex- 
periments was as follows: The plate and lens are first carefully to be 
freed from dust, grease, moisture, etc., and then to be screwed into 
their respective places at the center of the plate o 0’ and at the top of 
the plug Z as securely as possible. The screw stop A is lifted high 
enough that the plate case 0 0s may be pushed over the lens with- 
out danger of touching it. Then the miscroscope is adjusted and all 
parts eventually clamped in place with reference to the particular part 
of the glass plate where the test is to be made. It is expedient to 
clamp 4; first, and then to clamp S, in such a way that the point in 
question may occupy the middle of the field of view. During these 
operations it will have been necessary to gradually lower the stop A, 
though not quite as far as the position of contact. This is best done 
when the microscope is fixed. Under these circumstances, however, if 
the lever H H’ should not swing freely, the long arm H’ is presumably 
too heavy, and w’ must be moved further outward along the slide (a 
little weight r being added if need be) until the pressureless contact 
occurs, when the lever swings freely. This adjustment may actually 
be made to an accuracy of about 1 gram, a degree of sensitiveness far 
in excess of the demands. 
The load is now applied, beginning with the stirrup R as the first 
centerpoise. The screw B, which has thus far stood low, is raised until 
the load is carried simultaneously by ce’ and ec. 6 is then raised very 
gradually, however, until the load & has been quite transferred from 
K K’ to H H’, the observer availing himself meanwhile of the elasticity 
of the long lever. Indeed it is possible to raise B so uniformly that 
the area of the spot seen in the field of the microscope scarcely en- 
larges, so that the strain is imparted to the plate at a very slowly 
increasing rate. Should the weight of & (2278) be regarded too large 
as a first step, smaller weights may be attached with a string. Having 
read off the first diameter of the ring and noted the first load, the large 
lever is unloaded and the scale pan attached to the hook of the stirrup 
k. Then the lever H H’ is again loaded in the manner described, 
and the value of the diameter of the spot read off for the second load. 
This process is repeated as often as desirable, with the single addi- 
tional precaution that when the limits of elasticity are being gradually 
approached (determined from preliminary trials), the weights are added 
to the scale pan in smaller steps than at the outset. Eventually, there- 
fore, the values P and P (load and spot diameter) which correspond to 
rupture are obtained. The method described has many advantages; 
