This subject has, more recently been investigated by H. Morphy,
of Trinity College, Dublin. The refinement of a closed vessel at
uniform temperature, in which the ice is formed and the
experiment carried out, is introduced. Thermocouples give the
temperatures, not only of the ice but of the aluminium sleigh
which slips upon it under various loads. In this way we may be
certain that the metal runners are truly at the temperature of
the ice. I now quote from Morphy's paper
281
"The angle of friction was found to remain constant until a
certain stage of the loading, when it suddenly fell to about half
of its original value. It then remained constant for further
increases in the load.
"These results, which confirmed those obtained previously with
less satisfactory apparatus, are shown in the table below. In the
first column is shown the load, _i.e._ the weight of sleigh +
weight of shot added. In the second and third columns are shown,
respectively, the coefficient and angle of friction, whilst the
fourth gives the temperature of the ice as determined from the
galvanometer deflexions.
Load. Tan y. y. Temp.
5.68 grams. 0.36±.01 20°±30' -5.65° C.
10.39 -5.65°
11.96 -5.75°
12.74 -5.60°
13.53 -5.65°
14.31 -5.65°
15.10 grams. 0.17±.01 9°.30'±30' -5.60°
16.67 -5.55°
19.81 -5.60°
24.52 -5.60°
5.68 grams. 0.36±.01 20°±30' -5.60°
"These experiments were repeated on another occasion with the same
result and similar results had been obtained with different
apparatus.
"As a result of the investigation the following points are
clearly shown:—
282
"(1) The coefficient of friction for ice at constant temperature
may have either of two constant values according to the pressure
per unit surface of contact.