20 0. F. Brackett—Measurement of Power. 
A A New Device for Measuring Power; by C. F. 
pee 
Brackett, Physical Laboratory of the College of New Jersey. 
THE following account of a method of measuring the energy 
expended on or rendered by a dynamo- or a magneto-machine 
will be of interest to those who have to do with the production 
of eleetricity in the large way in Which it is now employed in 
the enterprises of the day. 
The machine is so supported, on uprights, that it can freely 
turn through a small arc of a circle whose center lies in the 
geometrical axis of the armature. The support may be effected 
by means of knife edges or by means of smooth cylindrical 
bearings, attached directly to the machine or to a cradle on 
which the machine rests. In the latter case the cradle is made 
adjustable so that the bottom or floor can be raised or lowered, 
thus permitting machines of different construction, when placed 
thereon, to be brought into proper positions as regards axis of 
revolution and points of support. When the machine, thus 
mounted, is set in rotation, with closed circuit, the mechanical 
couple set up between the armature and field magnets tends to 
make the latter revolve in the same direction with the arma- 
ture. The value of the couple, thus operative, and which we 
desire to know, will be known if we know the-value of the 
couple, equal and opposite in direction, which is required to 
hold the machine fixed in its position of equilibrium. A lever 
arm is fixed to the machine or cradle in a horizontal position 
and provided with a sliding weight of known value, sufficient 
to hold the machine fixed in its position of equilibriam when 
in the performance of its duty. ip e couple required can thus. 
be known in terms of lever arm and weight. We then only 
need to know the number of revolutions in a unit of time when 
we have all the data needed in order to compute the energy. 
| W denote the weight, L the lever arm and n the number 
of revolutions in a minute, we shall have: energy =22W Ln, 
as in the case of the well-known Prony brake. 
For purposes of accurate scientific inquiry, the field magnets 
alone may be mounted and balanced on knife edges, so as to 
turn freely like the beam of a common balance. By this plan 
all useless work is excluded from the account. Friction at the 
bearings and at the brushes do not in this case have any ten- 
dency to make the field magnets revolve. 
In the Physical Laboratory of this institution there are several 
machines having the construction here pointed out. The 
leave nothing to be desired in point of sensitiveness or accuracy 
in their indications. 
Princeton, Nov. 23, 1883. 
