[Illustration: Fig. 656.--Interior of Thompson watt hour meter (type C-6) showing armature, small commutator and gravity brushes. A spherical armature moving within circular field coils is the construction adopted in this meter. The armature is wound on a very thin paper shell, stiff enough to withstand the strain due to winding and subsequent handling. The wire composing the armature is of the smallest gauge consistent with mechanical strength. The field coils, as before stated, are circular, and are placed as near each other as possible, one on either side of the armature, with the internal diameter just sufficient to give the necessary clearance for the rotating element. This construction prevents magnetic leakage. Ribbon wire is employed for the field coils, thus economizing space and further carrying out the idea of concentration.] The torque of this motor being proportioned to the product of its armature and field currents, must vary directly as the energy passing through its coils. In order then, that the motor shall record correctly it is necessary only to provide some means for making the speed proportional to the torque. This is accomplished by applying a load or drag, the strength of which varies directly as the speed. Ques. Explain the operation of the Thompson recording wattmeter. Ans. There being no iron in either field or armature of the motor element, no considerations of saturation are involved. The torque or pull of the armature is dependent upon the product of the field and armature strength. The strength of the field--there being no iron--varies directly with the current in the field. Thus the strength of the field with 10 amperes flowing to the load is exactly twice the strength of the field with 5 amperes flowing to the load. The strength of the armature is dependent on the voltage of the system to which it is connected, the armature element of the meter being practically a voltmeter. There is, therefore, a torque or pull varying directly with the strength of the armature multiplied by the strength of the field, or, in other words, varying directly with the watt load, and except in so far as influenced by friction, the speed of rotation varies directly with the torque or pull. The currents generated in the disc armature consist of eddy currents, which circulate within the mass of the disc. Installation of Wattmeters.--The various types of wattmeter differ so widely either in mechanical details, or operating principles, that it is customary for manufacturers to furnish detailed instructions for the installation of their meters. Such instructions should be carefully followed in all cases, but the following will be found generally applicable to all types of motor meter: 1. After unpacking the meter, and before opening the case or cover, clean the latter carefully to remove all adhering particles of dust and excelsior.