DYNAMO-ELECTRIC MACHINES. IOI 



one lamp by the whole number, in such installation. Lamps also 

 vary in resistance nearly in the inverse ratio of the maximum amount 

 of light they are designed to furnish. A single lamp shedding a light of 

 10,000 candles, may, perhaps, for that reason, only oppose one tenth 

 of the resistance to the passage of a current, that might be offered if 

 the same quantity of light was distributed by 5 lamps. In incan- 

 descent lighting, usually employed, the resistance of one lamp is all 

 that has to be considered. The lamps being connected in multiple arc, 

 there is no more force required to overcome the resistance than that 

 opposed by one of them. Arc lamps can be connected in the same 

 manner, but are not, generally. The work done by a machine sup- 

 plying light by incandescence is also measured by the number of lights. 

 There is this important difference, however, between the two, that, 

 whereas, the resistance to be overcome in the one case is measured 

 by the sum of the resistances of all the lamps in circuit, that in the 

 other simply by rhe resistance of one, no matter how many may be 

 employed. The power in the first case is expended chiefly in pro- 

 ducing electro-motive force, in the latter current. These remarks 

 will give you some idea of the variations in the work to be accom- 

 plished, and also how desirable it is to construct machines that, under 

 all varying conditions, will preserve a constant electro-motive force, 

 or a constant current. 



This paper is long, but before closing, it may be profitable to 

 glance at the means usually adopted to overcome the opposing forces 

 to be met with, in the different purposes to which dynamo machines 

 can be applied. These considerations will also show you the necessity 

 of the knowledge of the work to be done by makers of these machines, 

 before they can determine the particular manner of construction. 

 Very high electro-motive force is obtained by winding the armature 

 with fine wire, and a high rate of speed. There is a limit in this 

 direction, owing to the resistance of the wire itself, which cannot be 

 exceeded. To obtain a large amount of current the opposite is neces- 

 sary, and the armature is wound with coarse wire. It may be said 

 here, that machines used for electro-chemical purposes can scarcely 

 be wound with wire that is too coarse, owing to the low electro-motive 

 force required. The electro magnets require to be wound in a corres- 

 ponding manner. Between the extremes of fine and coarse wire, 

 are to be found the sizes best adapted for the work to be done. 



