FEBRUARY 15, 1884.| 
The Thomson read the saine in both meas- 
urements, making the electromotive force 97.6 
volts. The divisions on the scale of the Ayrton 
and Perry were very small, making the reading 
quite difficult. From it were obtained, in the 
two measurements, 96.1 and 95.5 volts. 
Assuming that the field of the potential gal- 
vanometer is known, it is easy to determine that 
of the current galvanometer, as the magnets 
are interchangeable. <A series of observations 
was made in which a practically constant 
quantity was measured, first with one of these 
magnets on the magnetometer, and then with 
the other, alternating throughout the series. 
Twelve observations made in this way show 
a mean difference of 1.75% between the two 
magnetic fields. In the numbers used in these 
tests the difference is 2%. 
The current galvanometer was also compared 
with an Ayrton and Perry ammeter at the 
same time at which the potential instruments 
were compared. The circumstances did not 
allow the use of a stronger current than that 
passing through a single Edison lamp. The 
result was therefore not of great value. The 
Thomson showed 1.05 ampéres, and the Ayrton 
and Perry gave 1.03 for the same current. 
Several tests of the current galvanometer 
were made by means of a battery of five Grove 
cells, which were freshly set up. The reading 
of the galvanometer was noted, and then a 
resistance of one ohm was introduced into the 
circuit. The first reading was 19, and the sec- 
ond was 9.5 ; showing that the resistance of the 
battery and galvanometer was one ohm. ‘The 
electromotive force of the battery was then 
determined by means of the potential galva- 
nometer. Twomeasurements were made; the 
first giving 9.43 volts, and the second 9.56 
volts. Assuming the resistance to be one ohm, 
as shown above, these numbers would repre- 
sent, in accordance with Ohm’s law, the cur- 
rent in ampéres. ‘The current, as calculated 
from the galvanometer reading, was 9.5 am- 
péres. 
Many other tests of a similar character were 
made, all of which showed that the galvanome- 
SCIENCE. 
183 
ters must be admitted to be what they were 
assumed to be during the tests, — practically 
correct. But, even if they were somewhat in 
error, the similarity of conditions under which 
the competing systems were tested was such 
that all would be affected alike. 
Results. 
In determining the efficiency of the dyna- 
mos, after every thing was found to be in good 
order, a run of about half an hour was made; 
during which time readings were taken every 
two minutes, as nearly as could be, of 
the dynamometers and electrical instruments. 
Generally from ten to twenty sets of readings 
were secured. In most cases two or more 
‘runs’ were made; the repetition being in some 
instances the desire of the jury, and in others 
of the exhibiters. Sometimes the conditions 
under which the dynamo was running were 
changed by the exhibiters, with the expecta- 
tion, doubtless, of increasing its efficiency 
thereby. In the following summary of results, 
the numbers showing the electromotive force, 
current strength, power consumed, etc., are 
means of a number of observations. 
Photometry of arc-lamps. 
The table on the next page shows the results 
of the photometric comparison of the two are- 
lamps, and will be easily understood. The ar- 
rows show the direction of the light measured in 
each series: thus, <— means a horizontal meas- 
urement; \ means that the measurement was 
of the light going upward at an angle of forty- 
five degrees ; and ~ refers to the light going 
downward at an angle of forty-five degrees. 
For convenience, the intensity in terms of the 
standard (an Edison incandescent lamp) is 
multiplied by 1,000 before dividing by the 
number of Watts. 
It will be seen that the different lamps dif- 
fered from each other considerably in their 
efficiency. This was especially true of the 
Weston lamp, which was irregular in its ac- 
Efficiency of dynamos. 
| Thomson-Houston dynamo 
for are lighting. 
Weston dynamo 
for arc lighting. 
{ 
Weston dynamo Edison dynamo 
for incandescent lighting. for incandescent lighting. 
| Sept. 25. Sept. 26. 
Electromotive force, in volts . | 1232.0 1175.0 
Currentinampéres. ... 10.03 10.08 
Electrical horze-power | 16.6 15.9 
Horse-power consumed . 19.22 20.59 
Percentage of efficiency . 83.9 (Wie? 
Sept. 28. -——Sept. 29. -—— Oct. 2. 
626.0 69.2 60.0 65.0 124.9 122.8 
20.3 168.1 145.7 157.4 124.7 119.3 
17.0 15.6 inlet 13.7 20.9 19.6 
19.75 18.55 12.8 15.5 21.96 20.64 
86.5 84.1 91.4 88.4 95.2 95.0 
