3o8 



ELECTRIC LIGHTING. 



Tension and Quantity Machines. If, on a given dynamo- 

 electric machine, we expend a certain quantity of work w, 

 the expression of this work transformed into electricity may 

 be put into this form : w = Q E. Now this may be done 

 in these machines in two ways. 



By making Q very great and E very small, the machine, 

 having then little tension and supplying a large quantity of 

 current, takes the name of a quantity machine. 



Q may also be made very small and E very great; the 

 machine, having little quantity and a large electro-motive 

 force, takes the name of a tension machine. 



The former class should work with an external circuit of 

 small resistance ; the latter, on the contrary, requires a con- 

 siderable external resistance to satisfy the relations which 

 should exist between the external and internal circuits in 

 order to obtain the maximum utilizable effect. 



In conclusion, we give a table showing the different values 

 assumed by the electric elements of a machine, according as 

 it is constructed to supply a so-called quantity current, or a 

 so-called tension current. 



Elements of the working of Gramme machines, determined 

 by experiments made at Silver tow n, fitly, 1879. 



It will be seen that, according to this table, for the same 

 quantity of work expended, the electric elements, resistance, 

 intensity, and electro-motive force are notably different. 



