14 BULLETIN" 347, U. S. DEPAKTMENT OF AGBICULTUEE. 
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 (iV). 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 ( M ) 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 (L) 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. 
