182 
rapidly and continuously during the photomet- 
ric comparison. 
Neither of the two lamps under test illumi- 
nated equally in all directions. ‘They were 
therefore compared in nine different positions, 
each lamp assuming three, which were desig- 
nated respectively, ‘flat,’ ‘edgewise,’ and 
‘forty-five degrees ;’ and each position of one 
was compared with all of the other. Five sets 
of readings were made at each position, mak- 
ing, in all, forty-five comparisons of the two 
lamps. A number of preliminary comparisons 
were made, which were not considered as form- 
ing a part of the actual test. The latter was 
made on the evening of Sept. 29. 
The determination of the efficiency of the 
dynamos consisted in measuring the power 
consumed, as shown by the dynamometer, on 
the cradle of which the dynamo was placed, 
and at the same time measuring the current 
and the electromotive force at the binding- 
posts of the machine. The speed of the 
main shaft being nearly uniform, it was ne- 
cessary to place different pulleys upon it, in 
' order to secure the necessary speed for the 
armatures of the different machines. The 
speed of running being a matter which con- 
cerned the exhibiters rather than the jury, 
they were requested to furnish the dimensions 
of these pulleys, and accordingly did so. The 
average speed of the armature of the Weston 
dynamo for incandescent lamps was a little 
above ten hundred and thirty revolutions per 
minute, during three different series of obser- 
vations made while the machine was on the 
cradle. The Edison dynamo was placed on 
the cradle on the afternoon of Oct. 2, when a 
series of measurements was made with an 
average speed of ten hundred and sixty-eight 
revolutions. This was above what may be 
called the ‘normal speed,’ which was due 
partly to the size of the pulley, and partly to 
the fact that the engine was doing but little 
other work, and was probably running a little 
above its normal rate. In the evening the 
tests were continued, the speed of the arma- 
ture being a little below a thousand revolu- 
tions, the electromotive force being also less. 
It will be observed that the ‘efficiency’ of this 
dynamo, under the latter conditions, differs 
from that under the former by only two-tenths 
of one per cent. Particular attention is called 
to the fact, that no photometric measurements 
were made with lamps on the circuit of the 
Edison machine, which was on the dynamome- 
ter; those used being supplied from another 
similar dynamo, which was run by an Arming- 
ton and Sims engine, which formed a part of 
SCIENCE. 
[Vou. IIL, No. 54. 
the Edison exhibit. A glance at the results © 
given below will show that the electromotive — 
force in the latter case was much lower than 
in the former. 
Tests of the galvanometers. 
Although the jury was satisfied of the accu- 
racy of the Thomson galvanometers, within all 
practical limits, before deciding to rely upon 
their indications, for reasons that need not be 
referred to here, it was considered desirable, 
after the conclusion of the tests, to make such 
an examination of them as would leave no 
doubt as to the correctness of this opinion. 
The chief cause of error in these instruments, 
and in all of a similar construction, is the pos- 
sible variation in the strength of the permanent 
magnets which establish the field in which the 
needles move. The existence of a strong field 
is a great advantage, as has already been. 
pointed out, provided its value is known. An 
examination of the instruments was made be- 
fore they were taken to Cincinnati: and then, 
again, when they were mounted in the testing- 
room, they were compared with others not 
liable to such alterations, as before related. 
Numerous tests were made to ascertain if each 
instrument was consistent with itself by meas- 
uring the same quantity with the magnetome- 
ter at different points of its scale, thus varying 
the position of the needle; and the results 
were satisfactory. Finally, after the instru- 
ments were returned to the physical laboratory 
of the Ohio state university, they were sub- 
jected to tests, a brief account of which is as 
follows : — 
For the potential galvanometer, ten cells of 
the ‘ gravity battery,’ — the elements of which 
were zinc, zinc sulphate, copper sulphate, and 
copper, —in good condition, were individually 
compared by the condenser method with a 
standard Daniell cell set up for the purpose. 
They differed very little among themselves ; 
and when the electromotive force of the ten, in 
series, was measured by means of the Thom- 
son instrument, the resulting electromotive force 
of the Daniell was 1.106 volts. The instru- 
ment was also compared with one of Ayrton 
and Perry’s voltmeters, kindly furnished for 
the purpose by the Electric supply company of 
New-York City. For this purpose recourse 
was had to an Edison lighting-plant, the in- 
struments being applied to the same lamp. 
The fall in potential in the lamp was at first — 
110 volts, which was beyond the range of the 
Ayrton and Perry instrument; but it was re- — 
duced a little below 100 volts, and two meas- 
urements were taken with each instrument. 
