758 REPORT—1890. 
‘coefficient of abrasion’ of two kinds of matter would then be the ratio of the losses 
under similar circumstances, 
It seems highly probable that within wide limits this ratio would be indepen- 
dent of either the velocity or the pressure at which the comparisons were made; 
for it seems reasonable to suppose that in each case the weight abraded would be 
proportional, other things the same, to the pressure (from the laws of friction), simi- 
larly also to the velocity, for in each case the work done is so proportional. If 
this be assumed to be so, we have, at least, within wide limits, m= Be where m 
is the weight abraded over the area a under the pressure p in the time ¢, the sur- 
faces having the relative velocity v, / being the necessary equating constant, and 
might be well called the coefficient of abrasion, or the ‘ absolute coefficient of hard- 
ness,’ to distinguish it from Mohs’s scale. 
Thus the definition of ‘absolute hardness’ would be the reciprocal of the 
weight abraded over unit area under unit pressure in unit time, where the surfaces 
have relative unit velocity, or, combining the last two, per unit displacement. 
The value of & taken should be the final one; that is to say, the process should 
go on sufficiently long so as to reach a constant stage. Also it is necessary to sup- 
pose that by some means the abraded material is removed as it is generated. 
The total work may be divided into two parts—the heat <enerated and the work 
spent purely in disintegrating the material. It by no means follows, because the 
total work applied to produce a given displacement is proportional to the pressure, 
that the amount of material abraded is so proportional. For the parts may not 
always bear the same ratio to each other. However, within limits they probably 
do so, but the question, of course, is simply a matter for experiment. 
Some preliminary experiments have been carried out with the object of investi- 
gating this, but the apparatus used, which was only a modification of the original 
plan, proved unsuitable. It, however, afforded encouragement to pursue the investi- 
gation further, and at present an apparatus is being constructed for the purpose. 
The original plan consisted essentially of two cylinders of the material to be 
tested placed parallel, touching each other, which were to be rotated in the same 
direction, and to rub each other while being pressed together by a constant force. 
It is unnecessary to know the areas in contact, for the pressure being the force 
divided by the area, the area appears in both numerator and denominator. 
On account of expense in construction this plan was modified in the experi- 
ments made, the second surface being stationary and always completely covered by 
the rotating cylinder. It was chiefly through this that the experiments were un- 
satisfactory, for the abraded material could not be removed from under the sta- 
tionary surface, and a very fine powder, which acted as a lubricant, gradually 
collected. Were both surfaces to rotate, the cylinders could be continuously cleaned 
by brushes as they turned round. 
It is easy to see that the dimensions of / are the same as that of the square of a 
velocity 
K=[V"]=[L°T-] 
6. The Effect of Direct and Alternating Pressures on the Human Body.' 
By J. SwInsurye. 
A Wheatstone’s bridge, which measured the resistance of the patient under 
various pressures, was made up. The alternating currents were measured with a 
non-inductive wattmeter arranged as an ammeter, the pressure being taken with a 
hot wire voltmeter. The tests were taken from hand to hand, the hands being dry, 
or wet with dilute acid in the case of direct currents, and dry in the case of 
alternating. 
The maximum current taken was ‘04 ampére by a subject whose resistance is 
low. He could have taken more if available. All the resistances are much lower 
1 Hlectrician, September 19, 1890. 
