This last result is in accordance with the laws of friction.  We say that when solid presses on solid, for each pair of substances pressed together there is a constant ratio between the force required to keep one in motion over the other, and the force pressing the solids together.  This ratio is called"the coefficient of friction."The coefficient is, in fact, constant or approximately

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so.  I can determine the coefficient of friction from the angle of friction by taking the tangent of the angle.  The tangent of the angle of friction is the coefficient of friction.  If, then, the coefficient is constant, so, of course, must the angle of friction be constant.  We have seen that it is so in the case of metal on glass, but not so in the case of metal on ice.  This curious result shows that there is something abnormal about the slipperiness of ice.

The experiments we have hitherto made are open to the reproach that the surface of the ice is probably damp owing to the warmth of the air in contact with it.  I have here a means of dealing with a surface of cold, dry ice.  This shallow copper tank about 18 inches (45 cms.) long, and 4 inches (10 cms.) wide, is filled with a freezing ’mixture circulated through it from a larger vessel containing ice melting in hydrochloric acid at a temperature of about -18 deg.  C. This keeps the tank below the melting point of ice.  The upper surface of the tank is provided with raised edges so that it can be flooded with water.  The water is now frozen and its temperature is below 0 deg.  C. It is about 10 deg.  C. I can place over the ice a roof-shaped cover made of two inclined slabs of thick plate glass.  This acts to keep out warm air, and to do away with any possibility of the surface of the ice being wet with water thawed from the ice.  The whole tank along with its roof of glass can be adjusted to any angle, and a, scale at the

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raised end of the tank gives the angle of slope in degrees.  A weight placed on the ice can be easily seen through the glass cover.

The weight we shall use consists of a very light ring of aluminium wire which is rendered plainly visible by a ping-pong ball attached above it.  The weight rests now on a copper plate provided for the purpose at the upper end of the tank.  The plate being in direct contact beneath with the freezing mixture we are sure that the aluminium ring is no hotter than the ice.  A light jerk suffices to shake the weight on to the surface of the ice.

We find that this ring loaded with only the ping-pong ball, and weighing a total of 2.55 grams does not slip at the low angles.  I have the surface of the ice at an angle of rather over 131/2, and only by continuous tapping of the apparatus can it be induced to slip down.  This is a coefficient of 0.24, and compares with the coefficient of hard and smooth solids on one another.  I now replace the empty ping-pong ball by a similar ball filled with lead shot.  The total weight is now 155 grams.  You see the angle of slipping has fallen to 7 deg..