476 Brackett and Young—Edison’s Dynamometer, 
trials made to determine its efficiency. We found that the 
Prony registered 93°2 per cent of the power transmitted by the 
Edison. This result we consider quite reliable, the 6°8 loss being 
reasonably accounted for by friction of counter-shaft journals 
and the slip of the belt intervening between the instruments. 
To determine the efficiency of the dynamo-machine, we made 
three different tests. In all of them the power expended+in 
driving the revolving armature was measured directly by the 
Edison dynamometer. The small additional amount spent in 
maintaining the field of force was calculated from the measured 
resistance of the magnet coils (1-47 ohms) and the difference of 
electric potential between the terminals of the coils, measured 
by a high resistance galvanometer (Thomson). It was assumed 
in this calculation that the machine which furnishes this current 
was of about the same efficiency as the one experimented on, It 
being of similar construction. The formula is 
1:25 & 44°25 ft. lbs. x te x t, 
where ¢ is the duration of the experiment in minutes. V is the 
difference of potential in volts; r is the resistance or the coil 
‘47 ohms ‘25 is the number of foot lbs. of work done in 
one minute by an electromotive force of one volt driving 4 
current through one ohm; and finally 1:25 is a factor embody- 
ing the assumption that the efficiency of the machine, produc- 
ing the magnetizing current, is 80 per cent. The amount of 
this expenditure is trifling, not exceeding three per cent of the 
work used in driving the armature but its neglect might lead 
to misapprehension. 
On March 19th the current produced was measured by the 
electrolytic method. We employed copper electrodes present: 
ing opposed surfaces of about one square foot each, an ge 
about one inch apart in a solution of cupric sulphate. e also 
measured the resistance of the circuit by the bridge method 
both at the beginning and end of the experiment so as to take 
account of heating. For a check, we also measured the differ- 
ence of potential between the terminals of the machine. 
On April 8d we had recourse to the calorimeter, employing 
a resistance coil immersed in about 175 lbs. of water, the resist 
ance of the coil being such that the work done elsewhere in the 
circuit could be calculated and allowed for as a small fraction 
of that directly measured. The difference of potential between 
the terminals was also measured for a check, as on March 19th. 
The resistance of the armature was 0-14 of an ohm; that of the 
rest of the circuit was made to vary in the different exper 
ments, from 1°9 to 3°2 ohms. 
We shall use the expression available energy to denote the 
energy developed in that part of the cireuit which is outside of 
