232 
DR. J. HOPKINSON AND MR. E. WILSON ON 
of copper wire 3*5 miliims. diameter. The electromagnets are bolted to circular 
cast-iron frames (B), which serve also for supporting the bearings of the armature 
shaft. The centres of the 12 electromagnets on each frame are equally spaced out 
on a circle 8f inches radius concentric with the axle of the machine. Each cast-iron 
frame has a cross-sectional area of 4'8 sq. inches. The opposing pole pieces of the 
electromagnets have an air space of 1-|- inch between them, through which the 
armature coils rotate. The 24 electromagnet windings are coupled in series, and 
have a total resistance of 1*8 ohms, the normal exciting; current being about 
22 amperes. 
The armature of each dynamo consists of 12 coils or bobbins (C) with wooden 
cores (D) f inch thick. Each core is 7 inches long (radially), with rounded ends^ 
the outer being struck to a circle 3|- inches and the inner 1|- inches diameter. The 
ends of the respective coils are brought to a screw-plug commutator board fixed to 
the shaft, by means of which a series of combinations can be made. Each coil 
consists of 10 layers, 8 convolutions per layer, 2*2 miliims. copper wire, having a 
resistance (cold) of T87 ohm, and at a speed of 1000 revolutions per minute, with 
normal excitation and fully loaded, is intended to give 50 volts at its extremities. 
The full load current for each coil is lG'G amperes, so that at 1000 revolutions per 
minute, or a frequency of 100 complete periods per second, the machine should give, 
with its 12 armature coils in parallel, an output of 200 amperes 50 volts. The two 
terminal rings of the screw-plug commutator are connected by conductors to two 
gun-metal collector rings, insulated from one another and from the shaft by means of 
ebonite. Each collector ring is provided with two 1-inch copper wire brushes, 
carried by adjustable bar-holders fixed to the terminal blocks of the dynamo. 
The potential difference between any two points at any epoch is determined by 
means of a Kelvin quadrant electrometer and a revolving contact-maker fixed to the 
shaft of the dynamo. The contact-maker consists of a disc of gun-metal which 
carries two rings, one of gun-metal insulated from the disc, the other of ebonite. 
Into the latter is inserted a strip of metal inch wide, which is in permanent 
contact with the gun-metal ring. Two insulated brushes are attached to a movable 
