14 BULLETIN" 347, U. S. DEPARTMENT OF AGRICULTURE. 



are attached to castings at the top (I) near the base (H) of the 

 machine. The lower sprocket wheel is directly connected by a worm 

 gear to an electric motor (G). The sprocket chain is provided with 

 four small lugs (X), which, when the chain is being driven by the 

 motor, engage a spring bolt attachment which projects inward on 

 either side of the slot on the crosshead (M). This raises the cross- 

 head until tripped by a rod projecting downward from an upper 

 crosshead (L). When this takes place the crosshead (M) which 

 holds the electromagnet falls until it comes in contact with the 

 hammer (N). The electromagnet is supplied with current from the 

 circuit in series with an ordinary electric lamp which not only serves 

 to cut the voltage in the line but acts as a pilot lamp as well. This 

 current is conveyed through two small conducting rods running par- 

 allel to the guide rods, both of which are insulated from the rest of 

 the machine. The current passes from one of these conductor rods 

 through a small carbon brush to a make-and-break attachment on 

 the upper crosshead (L), thence down one of the guide rods, through 

 the electromagnet, and back to the other conductor rod. 



When the crosshead (if ) is raised by the lugs on the sprocket chain 

 and the current turned on the magnet the hammer is lifted until the 

 crosshead (M) comes in contact with the make-and-break on the 

 crosshead (£) and thus releases the hammer, which falls, striking a 

 plunger (0). The plunger is made of armor-piercing steel, with the 

 maximum temper at its lower end, which is spherical in shape. The 

 upper surface of the test piece, which rests on a counter anvil (W) 

 of hard steel, is in direct contact with and tangent to the spherical 

 end of the plunger at its center point. 



It will be observed that the blow as delivered through a spherical- 

 end plunger approximates as nearly as practicable the blows of 

 traffic. Besides this it has the further advantage of not requiring 

 great exactness in getting the two bearing surfaces of the test piece 

 parallel, as the entire load is applied at one point of the upper surface. 



The upper crosshead (L) is raised through any desired height by 

 means of the long revolving screw (K), which is geared at its lower 

 end to a dial (Q) , on which the height of the make-and-break attach- 

 ment, and therefore the height of the hammer drop, may be read 

 directly. By means of the revolving dial and screw the height of 

 the crosshead may be adjusted, by very close approximation, to 

 within 1 millimeter. 



In order to prevent the crosshead (M) which holds the electro- 

 magnet from striking too hard a blow on the hammer when falling, 

 a dashpot was first used, but it has been found that a few drops of 

 cylinder oil on the lower end of the guide rods completely eliminated 

 this difficulty. 



