8 Mr. A. Campbell on the Magnetic Fluxes in 



(10) Dolivo Voltmeter. — Here a thin wire of soft iron is 

 drawn down into a solenoid. The value given is for the hollow 

 core of the coil (including the wire). 



(11) Nalder Voltmeter. — In this a small piece of soft iron 

 moves in the magnetic field produced by a coil outside it. 

 The number in the table is for the middle of the space within 

 the coil. 



(12). Tangent Galvanometer. — When the deflexion =45°, 

 the resultant field = earth's field -=- cos 45° 

 =0*26 (in London). 



(13) Kelvin Astatic Mirror Galvanometer. — The galvano- 

 meter was made very sensitive and almost unstable by means 

 of the controlling magnet. The mean control field (for this 

 condition) was found by measuring its sensitivity and com- 

 paring it with that when the earth's field alone was used. 

 The deflecting field for 1° would be less than 0*00002, and 

 depends on the degree of astaticism of the suspended magnets. 



(11) Ever shed Ohmmeter — (Old type; polarized, astatic). 

 The number given is a rough approximation to the B due to 

 the shunt-coil alone at 100 volts. About the middle of the 

 scale the field due to the series-coil would have the same value. 

 (15) Ever shed Ohmmeter — (New type, with soft iron 

 needle). The B given is that due to the shunt-coil alone (at 

 200 volts). It is clear that the earth's field cannot introduce 

 much error. In any case the errors due to external fields 

 can be eliminated (as the makers direct) by reversing the 

 current and taking the mean of the readings. 



(16) Campbell Frequency-Teller. — The B is measured 

 between the vibrating strip and the attracting pole of the 

 electromagnet. It will be seen that only a quite moderate 

 field is required to throw the spring into strong vibration 

 w r hen it is adjusted to the right length for resonance. 



(17) Double Pole Bell Telephone. — The diaphragm was so 

 close to the poles of the permanent magnet as to form an 

 almost closed magnetic circuit. A small search-coil was 

 wound round one of the pole-pieces (area = 0*24 sq. centim.), 

 and the diaphragm was then laid in its place. A throw of the 

 galvanometer was got by pulling off both the diaphragm and 

 the search-coil. Thus the flux-density given refers to the 

 pole-pieces. 



(18) Ay Hon and Perry Variable Standard of Self-Induc- 

 tance. — With the pointer at 0*038 henry, the total flux within 

 the inner wooden bobbin was about 5200 for 1 ampere. 



(19) Self-Inductance Standard (L = 0'20 henry).— This 

 was a coil of 1158 turns of insulated copper wire (diameter 

 1*24 inillim.), the outer diameter of the coil being 22 centim. 



