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To take a reading with the extensometer the thin steel tongue A is caused to vibrate, and the divided head then turned till the point X just touches the hard steel knife-edge on the tongue as it vibrates to and fro. This has proved to be a most delicate method of setting the micrometer screw, as the noise produced and the fact that the vibrations are quickly-damped out indicate to ^V^mm. the instant when the screw is touching the tongue. After the load is applied a second reading is taken in a similar manner, and the difference in the readings gives directly the extension of the test-piece.

If the test-piece is of small diameter, the spring does not vibrate in so satisfactory a manner; the cause of this is the flexibility of the test-piece, the instrument itself vibrating as well as the spring. Still, very delicate readings can be taken by simply deflecting the spring with the finger, and noting the contact as it passes the point. No damage can be done by advancing the micrometer screw too far forward ; all that happens is that the point passes the knife-edge on one side or the other.

The standard instrument is for use on centres 100 mm -apart. A similar instrument for use on centres only two inches (50 mm.) apart is also made.

After thoroughly testing one of the 100 mm. instruments the National Physical Laboratory report:—

“ The instrument is evidently reliable to about the one “ thousandth part of a millimetre under ordinary conditions “ of test.”

Chronograph for Laboratory use.

A strip of Morse paper 25 mm. wide is drawn by means of a motor under three pens attached to the armatures of three electro-magnets. One of these electro-magnets can be connected to a clock making contact every second, the other two being used for recording the events under observation. The movement of the paper can be started or stopped whilst the motor is running.

J. 4240. J)

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