Lord Kelvin
"By Permission of the Berlin Photographic Co., New York."
Lord Kelvin (Sir William Thomson), (1824-1907). Professor of Physics, Glasgow University. Invented the absolute scale of temperature: also many practical electrical measuring instruments. The foremost physicist of the latter part of the nineteenth century.
Michael Faraday
"By Permission of the Berlin Photographic Co., New York."
Michael Faraday (1791-1867). Famous English Physicist. Made many discoveries in electricity and magnetism; "Greatest experimentalist of the nineteenth century."
301. The Magnetic Fields of Generators.—In the magneto, the magnetic field is produced by permanent steel magnets. In dynamos powerful electromagnets are used. The latter are sometimes excited by currents from some other source, but usually current from the armature is sent around the field coils to produce the magnetic fields. Dynamos are classified according to the manner in which the current is sent to their field coils.
Fig. 285.—A series-wound dynamo.
Fig. 286.—A shunt-wound dynamo.
Fig. 287.—A compound-wound dynamo.
A. The series wound dynamo (see Fig. 285) is arranged so that all of the current produced by the armature is sent through coils of coarse wire upon the fields, after flowing through the external circuit.
B. The shunt wound dynamo (see Fig. 286) sends a part only of the current produced through the field coils. The latter are of many turns of fine wire so as to use as little current as possible. The greater part of the current goes to the main circuit. If the number of lamps or motors connected to the main circuit is increased, the voltage is lessened which weakens the current in the field coils, causing a weaker field and still lower voltage, producing a fluctuating E.M.F. which is unsatisfactory for many purposes. This fault is overcome by
C. the compound wound dynamo. This dynamo has both shunt and series coils upon its fields. (See Fig. 287.) If more current is drawn into the main circuit with this dynamo, the series coils produce a stronger field compensating for the weaker field of the shunt coils, so that uniform voltage is maintained. The compound wound generator is therefore the one most commonly employed.