176 
The uprights bb form part of a rigid frame- 
work, well bolted together, one side of which 
is seen in outline on a smaller scale in fig. 3. 
A scale-beam, ff, made in the form of an 
inverted _, and graduated to fractions of an 
inch, had the lower extremity of its vertical 
arm fastened to the platform cc; while at the 
angle was an eye which was centred upon a 
by the screws iii. 
Hire. 1. 
It is readily seen that the platform, when it 
was raised from the ground, as shown in fig. 1, 
was free to swing through a limited space, 
either to the right or left, and that any eccen- 
tric weight or force so applied as to tend to 
swing it to the left could be compensated by 
a weight upon the scale-beam, ff, which swings 
with it: so that the platform could be still 
kept in its horizontal position. 
The axis about which the swinging tends to 
occur is the line of contact between aw and 
gg, which line we shall for brevity henceforth 
call the axis aa. 
ay ee ES 2 
LX SSAINV EE NN IW OS 
ee Set ae cell 
7 F q 
Fie. 2. 
Qa 
Hep 
ea 
In setting up the dynamometer, it was so 
placed that the axis aa was directly below the 
axis of the driving-pulley, and in the same ver- 
tical plane with it. The dynamo to be tested 
was placed upon the platform cc, while cc was 
resting upon the ground; and it was blocked 
up on cc to such a height, that the axis of the 
armature was as nearly at the same height as 
the axis aa as could be readily done by direct 
measurement. 
fected with all necessary exactness once for 
SCIENCE. 
This adjustment could be ef-. 
1 oe 
=, 
[Von. IIL, No. 54, 
all; for, even though the armature were slightly 
above or below the axis aa, no error would 
thereby be introduced into the observations. 
The axis of the armature was also set, as 
nearly as it could be conveniently, in line with 
the axis aa; but the final adjustment, so that 
the centre of the armature-pulley was neither 
to the right nor left of the axis aa, was made : 
mechanically as follows. The platform and 
dynamo were raised from the ground, and the 
girders gg carefully levelled by means of the 
jackscrews. The platform was then brought 
to a horizontal position by placing compen- 
sating weights upon it. The belt was next 
adjusted, and was tightened by lowering the 
platform. It was then found, that, in case 
the axis of the armature was to the right or left 
of aa, the tension of the belt exerted a force 
to tip the platform, and it no longer stood 
horizontal. This was corrected,—one half, by 
shifting compensating weights ; and the other 
half, by moving the dynamo to the right or 
left. The belt was then slackened by raising 
the platform, and a similar adjustment again 
made to bring the platform to the horizontal 
position. This was repeated until the plat- 
form stood horizontal, whether the belt was 
tight or loose; both girders being at the same 
height, and both accurately levelled, account 
being taken of the bending. 
Gaeoe 
A further application of compensating weights 
was employed to render the balance sensitive. 
With the Edison dynamo, which is top-heavy 
(i.e., its centre of gravity is above the axis of 
the armature), more than a ton was hung on the 
sides of the platform to bring the centre of 
gravity of the whole down to the axis aa. 
With the other dynamos, whose centres of 
gravity are below the axis of their armatures, 
compensating weights of smaller amount were 
placed upon a staging built on the platform 
above the dynamos, in order to bring the centre 
of gravity of the whole up nearly to the axis aa. 
In making the tests, the power was applied 
to turn the armatures clockwise in fig. 1. ‘This — 
caused the horizontal scale-beam / to rise; and 
weights were placed upon it to bring it back 
to the horizontal position. The moment om : 
; 
; 
