444 Proceedings of the Royal Society of Edinburgh. [Sess. 
the apparatus is set in motion. These toothed wheels are contained in an 
iron framework Y, fixed to the table. Fitting into the clamps A and B is 
the wire to which torsional fatigue is to be given, the length of wire taken 
being variable. In the farther side of the supporting steel rod is cub a little 
trench, and in this rests horizontally a long iron rod H C. At one end of 
this rod weights are attached, whilst at C is a clamp in a line above another 
one, D, fixed to one leg of the table. Between C and D is the wire to be 
given extensional strains. S is a semicircular strip of brass supported about 
a movable axis Q. The arms of this arc are loaded, and dip respectively 
into two iron cups, K being the one evident in the diagram. These cups are 
partially filled with mercury, the depth of which is so arranged as to 
ensure neither end of the brass arc being in contact when the apparatus is 
at its position of rest, and the arc being thus vertical. Projecting from, and 
clamped behind, slits on either side of the vertical support of the arc are 
two short brass rods N, which cause the oscillating pendulum to force the 
ends of the arc alternately into the mercury pools. Pieces of rubber tubing 
are fixed on these rods at the points of contact with the rod T. The leads 
from the battery enter at E, and a rheostat is introduced into the circuit so 
that the current is under control. From E one wire passes under the 
table and again emerges to form the coil G, which is surmounted by the 
two iron plates X, thence passing up the leg of the table and making con- 
tact by means of an iron bar fixed below the upper stance with the two 
mercury pools. The other wire from E passes to the key Z and thence 
upwards to the vertical rod at Q, a piece of wire inserted between Q and S 
ensuring the completing of the circuit. The mercury cups are fixed by 
screws to the table, and can be raised or lowered to any desired level. The 
pendulum is raised by hand, and allowed to oscillate. When M is at its 
maximum outward swing, the brass arc S is forced over until its end dips 
into the mercury at K. The magnetic action of G is then called into play, 
and M swings back, this time forcing over the arc to the other side by means 
of the projecting rod N, the end of the arc formerly dipping into the 
mercury thereupon breaking contact with the mercury and producing a 
spark. The pendulum now oscillates freely about the spindle P, alternately 
making and breaking contact with the two mercury pools. The current is 
regulated so that the pointer on A describes one complete revolution at each 
swing. If the current becomes too strong and the swings of the pendulum 
increase, the rod finally knocks against the contact maker at Z, causing it 
to be forced out of position, and thus breaking contact, so that the apparatus 
automatically prevents an increase in current causing the oscillations to 
exceed the normal maximum. If a wire be now clamped between A and B, 
