VARIOUS TYPES OF MACHINES. 703 



For a smaller velocity the machine discharges itself, whatever 

 be the current. For a velocity higher than this limit, the energy 

 absorbed nd 2 is greater than the calorific energy disengaged ; the 

 circuit becomes heated, whatever be the means of cooling, until 

 the resistance, which varies with the temperature, satisfies the 

 condition R = ?zC. 



If the machine with its circuit is kept in a bath which equalises 

 the temperatures, equilibrium is only possible for a certain tem- 

 perature which depends on the velocity. 



The intensity of the current which gives the temperature of 

 equilibrium depends itself only on the calorific energy Q lost, and 

 therefore on the method of cooling ; we have 



R 



3rd. In magneto-electrical machines, the term CI, which cor- 

 responds to the electrodynamic action, is generally very small 



1! 



Fig. 252. 



compared with the action of the magnetised pieces; the function 

 <j> (I) is first proportional to the intensity I for weak currents, and 

 tends to become constant for very powerful currents 



1259. Let us consider as an example a rectilinear conductor AB 

 (Fig. 252) of length 20, movable about a vertical axis passing 

 through its middle, and the bent ends of which dip in a ring-shaped 

 bath of mercury A'B' ; the mercury is connected with the point O 

 by another conductor, so as to form a circuit of resistance R. 



The . only effective force is the vertical component of the ter- 

 restrial field. If the rotation is uniform, and makes n turns in a 

 second, the flow of force cut by each branch in unit time is 

 or for the two 



If the total current is I, or - in each branch, the work necessary 

 f r keeping up the motion is 7rna 2 ZI, and the electromotive force 



