Mr. A. Campbell on the Magnetic Fluxes in 

 Table I. 



No. 



Name. 



Resistance, 

 Ohms. 



Full 

 Load. 



B (mean). 



1. 



2. 



3. 



4. 



5. 

 6. 



7. 

 8. 



9. 

 10. 

 11. 

 12. 

 13. 



14. 

 15. 



16. 



17. 

 18. 



19. 



Siemens Electrodynamometer 



Kelvin Ampere-Balance 



0-53 



0-0156 



0-58 



0-0078 



324 



74 



481 



0-00013 



30 



7X10- 5 



56 



586 



13,000 



(500) 

 126 



106 

 10 



4 amps. 

 20 amps. 

 10 amps. 



50 amps. 



0-2 volt 



3 volts 



100 amps. 



18 volts 



500 amps. 

 15 volts 

 40 volts 



Defl.=45° 



(100 volts) 

 (200 volts) 

 (0 - 2 amp.) 



(1 amp.) 

 (1 amp.) 



80 

 18 

 65 



55 



450 



870 

 400 

 700 



14,200 



580 



75 



70 

 0-26 



0-008 



(about 0-2) 



(about 10) 



280 



3000 



20 

 46 



Bifilar Mirror Wattmeter (after ] 

 Dr. Fleming). J 



Ayrton and Mather D'Arsonval 1 

 Galvanometer. J 



Weston Voltmeter 



Davies Voltmeter (Muirhead) ... 



Ayrton and Perry Magnifying! 

 Spring Voltmeter. j 

 Richard Recording Ammeter 



Nalder Voltmeter 



Any Tangent Galvanometer 



Kelvin Astatic Mirror Gralvano- \ 



meter. J 



Evershed Ohmmeter, Old Type . . . 



,, ,, 'New Type... 



Campbell Frequency-Teller 



Bell Telephone (double pole) 



Ayrton and Perry Variable In- ] 

 ductance Standard. J 

 Standard Inductance Coil "1 

 (L= 0-2 henry. J 



As the numbers in the above Table throw an interesting 

 light on the behaviour of many of the instruments it seems 

 desirable to discuss them more fully in order. 



(1) Siemens Electrodynamometer. — Measurements taken at 

 the middle and the top of the swinging coil (of 4 turns) in 

 direction perpendicular to the plane of the fixed coils gave 

 B = 120 and 40 respectively for the thin coil, and B=21 and 

 16 for the thick. It will be seen that when the thick coil is 

 used the deflecting field is quite comparable with the earth's 

 field. 



This, of course, introduces an error with direct currents 

 unless care is taken to place the instrument so that the 

 direction of the earth's field is at right angles to that of the 

 deflecting field, and in the proper sense, i. e. with the instru- 

 ment looking east or west according to the direction of the 

 current in the swinging coil. The above results show that 

 the maximum variation at 15 amps, introduced by wrono- 

 placing (viz., due to a field equal to twice that of the earth, 

 or 0*36) would be about 2*5 per cent, of the mean deflecting 

 field. This was verified by placing the electrodynamometer on 

 a well-levelled turntable, and connecting its thick coil with a 



