[Illustration: FIG. 40.--Impact testing machine.] The weight is lifted by a chain, one end of which passes over a sprocket wheel in the hoisting mechanism. On the lower end of the chain is hung an electro-magnet of sufficient magnetic strength to support the heaviest striking weights. When it is desired to drop the striking weight the electric current is broken and reversed by means of an automatic switch and current breaker. The height of drop may be regulated by setting at the desired height on one of the columns a tripping pin which throws the switch on the magnet and so breaks and reverses the current. There are four striking weights, weighing respectively 50, 100, 250, and 500 pounds, any one of which may be used, depending upon the desired energy of blow. When used for compression tests a flat steel head six inches in diameter is screwed into the lower end of the weight. For transverse tests, a well-rounded knife edge is screwed into the weight in place of the flat head. Knife edges for supporting the ends of the specimen to be tested, are securely bolted to the base of the machine. The record of the behavior of the specimen at time of impact is traced upon a revolving drum by a pencil fixed in the striking head. (See Fig. 41.) When a drop is made the pencil comes in contact with the drum and is held in place by a spring. The drum is revolved very slowly, either automatically or by hand. The speed of the drum can be recorded by a pencil in the end of a tuning fork which gives a known number of vibrations per second. [Illustration: FIG. 41.--Drum record of impact bending test.] One size of this machine will handle specimens for transverse tests 9 inches wide and 6-foot span; the other, 12 inches wide and 8-foot span. For compression tests a free fall of about 6.5 feet may be obtained. For transverse tests the fall is a little less, depending upon the size of the specimen. The machine is calibrated by dropping the hammer upon a copper cylinder. The axial compression of the plug is noted. The energy used in static tests to produce this axial compression under stress in a like piece of metal is determined. The external energy of the blow (_i.e._, the weight of the hammer X the height of drop) is compared with the energy used in static tests at equal amounts of compression. For instance: Energy delivered, impact test 35,000 inch-pounds Energy computed from static test .26,400 " " Efficiency of blow of hammer .75.3 per cent. _Preparing the material_: The material used in making impact tests is of the same size and prepared in the same way as for static bending and compression tests. Bending in impact tests is more commonly used than compression, and small beams with 28-inch span are usually employed.