The cores
have a projection opposite to the spindle, ab, which latter is
screw-threaded. By a relay one or other electro-magnet is put in
action, and the rotating core, which is magnetized, causes rotation of
the spindle by attraction, resulting in the movement of the contact
along the resistance stops. The relay is acted upon directly by the
potential of the dynamo, and the variable resistance is included in
the shunt field of the machine, so that changes in the potential,
resulting from changes in load or speed, are compensated for.
The arrangements of the lamp circuits and the lamp itself may now be
described. The lamps are all run in parallel circuit, but are divided
into groups of five, each group being controlled by a separate switch
on the board--Figs. 11 and 11A. These switches are not in
direct communication with the dynamo, but make that connection through
a large central switch, S2, which therefore carries the whole
current. The returns from each group are brought to the connections
seen between the two resistances, where the circuits may be
disconnected if desired, and the main current then passes through the
ammeter, A3, to the other terminal of the machine. One of the smaller
switches at the top, Fig. 11A, is directly connected with one terminal
of the 20 horse power dynamo before mentioned, and the other side of
the switch to the motor in the machine tool exhibit.
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