DYNAMO-ELECTRIC MACHINES. 91 



constant current, is in the ratio of the square of the cross-section of 

 such magnet. In other words, if the electro-magnet is round, its 

 magnetic effect is in the ratio of the square of its diameter, for a con- 

 stant current and the same number of tnrns of wire. If the same 

 number of turns of a coil are spread over a great length there will be 

 the same magnetic effect generally, but it will not be so concentrated 

 as if the turns were closer together, or coiled in layers one over the 

 other. By increasing the sectional area of the electro-magnet there 

 is greater magnetic effect, but the magnetism will not be so intense ; 

 there will be more uniformity of field however. And this is 

 what Edison seems to have done in his machine. He uses very long 

 bobbins, ending in large masses of iron, which produce a more uni- 

 form field. This is undoubtedly better, owing to the sensitiveness of 

 the lamps. When incandescent lamps are heated to nearly the max- 

 imum, it is important that the dynamo feeding them, should supply 

 as uniform a current as possible. There is a medium length for elec- 

 tro-magnet coils, which practice only can give. If the coil be too long 

 the magnetism is diffused over length. If it be too short, there will 

 be a loss of magnetic effect of the current from the great distance of 

 the outer turns from the iron. A long coil may be useful for some 

 purposes where a short one with the same number of turns would not 

 answer. The length of the coil is somewhat dependent upon the 

 position the magnet will occupy in the machine. It is an essential 

 feature, however, in all dynamo-electric machines that the magnets 

 should be as close together as possible. Having arranged the electro- 

 magnets to form the field, the next consideration is the winding of 

 the armature. The various methods by which this can be done are 

 almost endless. There is one general direction, however, in which the 

 wire must be placed, as it moves in the field, to induce a current. If 

 the motion of the wire is parallel with its length there will be very little 

 current generated, because it will cut very few lines of force. If the 

 wire moves in a direction parallel to the lines of force, there will be 

 no current generated, because in that case, it would not cut any lines 

 of force. It is therefore essential that the wire encircling an armature 

 should be so wound as to cut as many lines of force as possible. To 

 do this, the armature should be placed so that its axis would be in 

 the centre of the field, and the field should be so formed that its lines 

 of force would be like radii, or arms projecting radially from the axis 



