A MAGNETIC CrRVli TRACIili 



21 



fluxmeter in which the suspended coil has its restoring torque counter- 

 balanced for all deflections within a range sufficient for accurate 

 delineation of magnetic curves. 



Description of the Apparatus 



The operation of the apparatus is as follows: a long, sensitive, photo- 

 electric cell is fitted with a V'-shaped slit, as shown in Fig. 1 ; a beam of 



LAMP 



/ 



FLUXMETER 



i— &- 



EMF 

 Fig. 1 — The photoelectric cell circuit. 



light is reflected from the mirror of the fluxmeter and focused on the 

 slit of the photo-electric cell, which is connected, in series with a 

 source of e.m.f., across the terminals of the fluxmeter; the e.m.f. is 

 adjusted once for all to such a value that, if the beam is at rest when 

 at the narrow end of the slit, at any other position the current con- 

 trolled by the cell will develop a torque in the fluxmeter coil which just 

 balances the restoring torque of the suspension. The fluxmeter 

 deflection will then be proportional to the change of flux which has 

 occurred within the search coil S. It may be found necessary to shape 

 the slit empirically to correspond to the unequal sensitivities of the 

 photo-electric cell at different spots. The fluxmeter used is a Leeds 

 and Northrup type 2290 HS galvanometer. It has a critical damping 

 resistance of about 100,000 ohms, and when used with about one 

 hundred ohms in the external circuit it is much over damped. 



The apparatus for registering the deflections photographically, and 



