316 
vitiates the results, introducing stresses into 
the test-piece which produce an effect not to 
be calculated upon. 
By referring to fig. 2, a device for enabling 
the machine to automatically centre the test- 
piece may be understood. The top and the bot- 
tom crossheads have in their centres a large 
spherical concavity. This concavity contains 
a segment of a sphere in which the wedges 
for griping the test-piece are placed. The 
spherical segment is made of steel turned and 
polished, and the concavity is lined with the 
best anti-friction metal. Any eccentric stress 
swings the segments in their sockets, and causes 
the axis of stress in the machine to coincide 
with the axis of the test-piece. The spherical 
segments weigh about two hundred pounds. 
They are, however, carefully supported on 
India-rubber springs, so as to eliminate as far 
as possible the weight of the segment from the 
friction in its socket. But supposing, under 
the most unfavorable circumstances, the whole 
weight of the segment does come on the joint, 
the coefficient of friction is not over two per 
cent: consequently a maximum cross-strain of 
four pounds on the test-piece will cause the seg- 
ment to swing, and to adjust itself to the axis 
of stress through the piece. As this weight of 
four pounds is less than half the least reading 
of the poise, it may be assumed to produce no 
sensible effect on the piece to be examined. 
The most of the testing-machines now in 
use require a careful preparation of the test- 
piece previous to an examination. If, for ex- 
ample, it is wished to ascertain the strength of 
an I-beam or of a channel, it is necessary to 
send the shape to the machine-shop, and plane 
a piece of one or two inches in area. ‘This re- 
quires much time and expense. ‘The specimen 
is then sent to the testing-machine and broken ; 
and what is obtained? Simply the result of a 
piece cut from the shape, which may or may 
not give a fair knowledge of the actual strength 
of the member in question. What is wanted 
at the present time is not the strength of a 
carefully prepared test-piece, broken under spe- 
cial circumstances, but of the actual bar just as 
it comes from the rolls in the mill itself. The 
spherical segments in the crossheads of the 
Fairbanks testing-machine have four sides in- 
clined at an angle of about twelve degrees to 
the axis of the machine. Two of these sides 
are curved, and two are straight. By using a 
number of wedges with backs correspondingly 
curved or straight, any piece, of whatsoever 
section, may be completely surrounded by the 
wedges, and griped on all sides; so that a 
channel, an angle, and I-beam, a T or a star, 
SCIENCE. 
[Vou. III, No. 58. 
or, indeed, any of the shapes now rolled in the 
mills, may be placed in the machine and broken 
in full size. 
Much time and labor have been spent to 
accomplish the power of autographically: re- 
cording, at each instant of time during the ex- 
periment, the amount of stress, and the effect 
produced thereby on the specimen. To the 
best of the author’s knowledge, Professor 
Thurston of the Stevens institute was the first 
to originate the idea of making a testing-ma- 
chine in such a manner as to record graphically. 
In 1876, at the Centennial exhibition, Professor 
Thurston exhibited a machine designed to 
make tests in torsion and to record the action 
thereof. As a matter of history, it may be 
stated, that, while engaged in examining mate- 
rial for the East River bridge in 1877, the au- 
thor designed and built the first testing-machine 
to autographically record results of the experi- 
ments in other stresses than that of torsion. 
While this machine, being the first of its kind, 
was necessarily crude and imperfect, it gave 
for some years very satisfactory results, and is 
still in use by the Bridge company. While the 
present machine is essentially different from 
the one just mentioned, the principles employed 
are the same as those devised for the East 
River bridge. 
Referring to fig. 2, it will be seen that the 
battery G is attached to the top of the adjust- 
ing-screws hh. These screws are carefully 
insulated from the rest of the machine. As 
soon as the test-piece is placed in the top cross- 
head, it becomes thereby connected with the 
battery. On the lower end of the specimen 
may be seen a small clamp, carrying an electro- 
magnet. One end of the wire of this magnet 
is in connection with the specimen, while the 
other end of the wire is joined to a little bind- 
ing-screw on top, to which the other pole of 
the battery is attached; so that the current 
actuating this magnet flows through the test- 
piece under examination. It will also be seen 
that the magnetic clutch, m, for holding the 
driving-belt on the tight pulley, is also included 
in this part of the battery-circuit. When the 
rupture of the test-piece occurs, the current is 
broken, the magnetic clutch is released, the 
belt slides by means of the counterpoise weight 
to the loose pulley, and the testing-machine 
stops. On the top of the specimen nearest to 
the upper crosshead is attached a second clamp, 
carrying a small sheave or pulley. Around 
this pulley, parallel to the specimen, and at- 
tached to the armature of the lower clamp 
magnet, passes a flexible steel tape, y, that, 
after passing alongside the specimen, runs 
