316 Prof. J. A. Fleming and Mr. J. E. Petavel 



o 



measuring trie mean value of the power supplied to the arc. 

 This was accomplished by the use of a suitable non-inductive 

 resistance and a bifilar reflecting wattmeter, the series coil 

 of which was in the circuit of the arc, and the shunt coil 

 of which was connected across the carbons of the arc. The 

 following is a description of this wattmeter : — 



The series coil was wound with 10 turns of thick copper 

 wire (No. 10 S.W.Gr.) and had a resistance of *01 ohm. The 

 shunt coil was wound with 23 turns of thin wire, and had a 

 resistance of 3" 76 ohms, and was placed in series with a 

 non-inductive platinoid resistance of 1200 ohms The shunt- 

 coil was suspended by two fine silver wires, which also served 

 to conduct the current in and ont of the coil. The con- 

 trolling and deflecting forces so balanced each other at small 

 displacements as to make the angular displacement of the 

 movable coil very nearly proportional to the power passing 

 through the wattmeter. The movable coil was provided with 

 a mirror by means of which the image of a wire illuminated 

 by an auxiliary arc lamp was reflected on to a fixed scale. 

 This scale was carefully graduated, so as to read directly in 

 watts. It was found that the earth's magnetic field caused a 

 small deflexion of the movable coil when using continuous 

 currents and when the shunt current was passing through it, 

 when at the same time no current was flowing in the series 

 coil. This terrestrial field was neutralized by magnets, or 

 else the scale was shifted so that the part of the deflexion of 

 the shunt coil due to the terrestrial field was eliminated. The 

 best way would have been to have turned the wattmeter 

 round through a certain angle, but as it had to be screwed 

 up against the wall in a fixed position for steadiness, it was 

 found that the above method was the simplest plan for obviating 

 this source of error. The vibrations of the movable coil were 

 damped by means of a mica vane dipping into a dash-pot 

 filled with oil. When all the adjustments were made, it was 

 found that this wattmeter produced a deflexion of the spot of 

 light on the scale almost exactly proportional to the power 

 passing through the instrument. This wattmeter was then 

 connected up to the arc lamp, so that the whole current 

 actuating the arc lamp passed through the series coil, the 

 terminals of the shunt coil being connected to the carbons 

 of the arc. 



A series of preliminary experiments were then made for 

 the purpose of enabling us to eliminate from the wattmeter- 

 readings, firstly, the power taken up in the wattmeter itself ; 

 secondly, the power taken up in the shunt coil of the arc- 

 lamp and other shunt, resistances ; and, thirdly, the power 



