96 DYNAMO-ELECTRIC MACHINES. 



which for 16 lights will probably average 10 or more ohms. Experi- 

 ment has shown that the total resistance, including machine, 16 

 lamps and conductor, was 83.51 ohms. The electro-motive force in 

 Volts was equal to 839, which divided by the total resistance, gave 

 a current of 10 and a fraction amperes. The total resistance was 

 found by adding that of the machine to the 16 lamps, which were a 

 little over 4^ ohms each, and that of the conductor from the machine 

 to the lamps, together. The very high electro-motive force developed 

 by such a machine makes it very dangerous to handle. It is neces- 

 sary to run the machine at a very high rate of speed to obtain this. 

 While European makers run at velocities varying from 350 revolu- 

 tions for large machines, to 1500 for the smallest size, few if any of 

 the Brush machines run at a less velocity than 1400 revolutions. 

 In addition to the destructive effect resulting from so high a speed 

 there is another, and perhaps more serious one still, in the great 

 amount of sparkling which is constantly visible, while the machines 

 are running, at the commutators. This is in part owing to the high 

 electro-motive force and to the high speed, but chiefly to the con- 

 struction of the ring and his mode of collection. The Brush arma- 

 ture, as you are probably aware, is made up of coils varying from eight 

 to sixteen in number. Each coil is connected at the inner end to 

 the one diametrically opposite, and the outer end to the commutator, 

 which consists of but two pieces, with an intervening space between 

 each. He has, therefore, one commutator for each pair of coils. 

 There will be consequently, as many commutators as there are pairs 

 of coils on the ring. A very irregular current is the result. If the 

 action of each commutator is analyzed it will be found that at one 

 portion of the revolution there is a very feeble current which in- 

 creases to a maximum, which again diminishes becoming feeble as at 

 first, and finally for a portion of the revolution, disappears altogether. 

 The result of these variations is a continuous adjustment of itself be- 

 tween the commutators, a portion from one at the maximum being 

 absorbed by another, which is only supplying a minimum of current. 

 That portion of the commutator which is cut off from the generating 

 coils twice in each revolution, positively stops all current. You will 

 therefore understand, that these variations in intensity, from zero up 

 to 2000 volts, several times in each revolution, multiplied by 1400, in 

 in each minute of time, must have a very destructive effect upon the 



