PRACTICAL OPERATION OF DYNAMOS. 205 



The Thomson meter is a small electric motor without iron, the 

 armature and field coils of which constitute an electrodyna- 

 mometer. It is connected to the mains and to the receiving cir- 

 cuit in the same way as the wattmeter shown in Fig. 122. The 

 field coils, BB, Fig. 1 24, of this motor correspond to the coil B, 

 Fig. 122, and the armature A and non-inductive resistance^?, 

 Fig. 1 24, correspond to the coil A and the resistance R of Fig. 

 122. Current is led into the rotating armature A by means of 

 the metal brushes dd which rub very lightly upon a small silver 

 commutator e. The armature A is mounted on a vertical spindle 

 which is as nearly frictionless as possible, and the opposition to 

 rotation is due mostly to the electromagnetic drag of the per- 

 manent steel magnets MM upon the copper disk / which is 

 attached to the armature spindle. 



The integrating action of the watt-hour meter is as follows : In the first place, 

 the armature rotates at a speed which is proportional to the driving torque, because 

 the retarding torque with which the magnets MM act upon the disk/, is proportional 

 to the speed. In the second place, the driving torque is proportional to El exactly 

 as in the case of the force action between the coils of a wattmeter as explained in 

 Art. 91. Therefore, the speed of the armature is proportional at each instant to the 

 power delivered ; that is the revolutions per hour of the spindle is proportional at 

 each instant to the watt-hours of work delivered per hour, so that the total number 

 of revolutions of the spindle in a given time is proportional to the watt-hours of work 

 delivered during that time. 



The starting coil.ln the above discussion it is assumed that the torque which 

 opposes the rotation of the armature is proportional to the speed. This is not strictly 

 true inasmuch as the retarding torque consists of two parts, namely, mechanical fric- 

 tion, which is nearly constant irrespective of speed, and electromagnetic drag which is 

 strictly proportional to the speed. If a constant driving torque in addition to the 

 torque due to the action of the two coils A and B could be provided to overcome 

 mechanical friction, then the speed of the meter would be proportional to the deliv- 

 ered power, and the indications of the instrument would be more nearly correct 

 This is accomplished in the Thomson meter by using an auxiliary field coil having a 

 few turns of fine wire connected in circuit with A and R, Figs. 122 and 124 This 

 is called the starting coil, and if the electromotive force between the mains is nearly 



tant, the current through this starting coil and through A and R is nearly con- 

 tant, and an approximately constant driving torque is produced by the electromagnetic 

 action of the starting coil on the armature A. 



93. Fuses and circuit breakers. A fuse is a link of fusible 

 metal placed in an electric circuit so that when the current ex- 



