=300. The Magneto and the Dynamo.=--Magnetos are used to _develop small_ currents, such as are used for telephone signals, and for operating the _sparking_ devices of gasoline _engines_. They are therefore found in automobiles containing gasoline motors. The most important device for producing electric currents by electromagnetic induction, however, is the _dynamo_. It is employed whenever large currents are desired. The principle of this device is similar to that of the magneto except that it contains an _electromagnet_ for producing the magnetic field. Since the electromagnet can develop a much stronger field than a permanent magnet, the dynamo can produce a higher E.M.F. and a much larger current than the magneto. [Illustration: 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.] [Illustration: 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. [Illustration: FIG. 285.--A series-wound dynamo.] [Illustration: FIG. 286.--A shunt-wound dynamo.] [Illustration: 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