used. (See Fig. 29.)

The weighing platform rests upon knife edges carried by primary
levers of the weighing apparatus, the fulcrum being on the bed
of the machine, and any pressure upon it is directly transmitted
through a series of levers to the weighing beam. This beam is
adjusted by means of a poise running on a screw. In operation
the beam is kept floating by means of another poise moved back
and forth by a screw which is operated by a hand wheel or
automatically. The larger units of stress are read from the
graduations along the side of the beam, while the intermediate
smaller weights are observed on the dial on the rear end of the
beam.

The machine is driven by power from a shaft or a motor and is so
geared that various speeds are obtainable. One man can operate
it.

In making tests the operation of the straining screws is always
downward so as to bring pressure to bear upon the weighing
platform. For tests in tension and cleavage the specimen is
placed between the top of the stationary cage and the movable
head and subjected to a pull. For tests in transverse bending,
compression, and cleavage the specimen is placed between the
movable head and the platform, and a direct compression force
applied.

Testing machines are usually calibrated to a portion of their
capacity before leaving the factory. The delicacy of the
weighing levers is verified by determining the number of pounds
necessary to move the beam between the stops while a load of
1,000 pounds rests on the platform. The usual requirement is
that ten pounds should accomplish this movement.

The size of machine suitable for compression tests on 2" X 2"
sticks or for 2" X 2" beams with 26 to 36-inch span has a
capacity of 30,000 pounds.

SPEED OF TESTING MACHINE

In instructions for making static tests the rate of application
of the stress, _i.e._, the speed of the machine, is given
because the strength of wood varies with the speed at which the
fibres are strained. The speed of the crosshead of the testing
machine is practically never constant, due to mechanical defects
of the apparatus and variations in the speed of the motor, but
so long as it does not exceed 25 per cent the results will not
be appreciably affected. In fact, a change in speed of 50 per
cent will not cause the strength of the wood to vary more than 2
per cent.[58]

[Footnote 58: See Tiemann, Harry Donald: The effect of the speed
of testing upon the strength and the s