. A text-book of radiology . ates in it, there being one complete cycle for eachrevolution. The Dynamo.—It is a fair estimate that over 99 %of the electricity used is obtained from dynamos. Thedynamo is a device for converting mechanical into elec-trical power. It consists of three essential parts—(a)The field magnet; (b) The armature; and (c) The com-mutator and collecting brushes. DYNAMO MACHINES. 19 (a) The field magnet, which generally forms part of theframework of the machine, is usually an electro-magnet,the poles of which have their opposing faces hollowed outto the arc of a circle, in
. A text-book of radiology . ates in it, there being one complete cycle for eachrevolution. The Dynamo.—It is a fair estimate that over 99 %of the electricity used is obtained from dynamos. Thedynamo is a device for converting mechanical into elec-trical power. It consists of three essential parts—(a)The field magnet; (b) The armature; and (c) The com-mutator and collecting brushes. DYNAMO MACHINES. 19 (a) The field magnet, which generally forms part of theframework of the machine, is usually an electro-magnet,the poles of which have their opposing faces hollowed outto the arc of a circle, in which space the armaturerevolves. When the field magnet is excited this spacewill be the seat of a powerful magnetic field. Theessential point about the field magnet is that its polesnever change; one is always north and the other is south. (b) The armature consists of an iron core, upon whichare wound one or more closed conductors, and is sodesigned as almost completely to fill the space between ARMATURE BRUSHCOMMUTATOR. ARMATUREFIELD MAGNET FIELD MAGNET COIL Fig. 5. Parts of a 2 Pole Dynamo. the field magnet poles, leaving just sufficient room for itto revolve without touching. It has been shown that torotate a closed conductor in a magnetic field is to set upan alternating current in it—so to rectify this alternatingcurrent the third essential part is devised. (c) The commutator. This consists of a number ofcopper bars mounted in the form of a cylinder, and insu-lated from the shaft and from each other. There are asmany bars as there are coils on the armature. Thebeginning of one coil and the end of the coil just pre-ceding it are joined together, and the two attached to acommutator bar. Two brushes of copper gauze or carbonare arranged to touch the commutator at opposite ends ofa diameter; and as the armature is revolved the com- 20 TEXT-BOOK OF RADIOLOGY. mutator bars pass in succession under the brushes whichcollect the current from them. Most dynamos which havea co
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