POLARISING RECEIVER 17 



if ju, be the refractive index of a substance, and $ 

 the angle of polarisation for that substance, the 

 relation holds good. 



fji = tan (/>. 



If a liquid substance contained in a tube BC 

 (Fig. 4) be subjected to a field produced by a coil 

 through which current is passing, its refractive 

 index will be changed ; hence </> will be changed 

 also. If rays of monochromatic light from a 

 lamp L pass through one nichol prism NI, then 



<D 



FIG. 4. 



through the tube BC of liquid (carbon disulphide), 

 afterwards passing through the analysing prism N 2 

 and thence to a photographic film attached to a 

 receiving drum D, the nichols being set at the polar- 

 ising angle, no light would reach the film unless a 

 current passed through the coil round BC. The 

 terminals of such a coil would be connected to the 

 line AA, either directly or through a relay, and 

 hence the film on D would record the impulses sent 

 through the coil. The sensitiveness of such an 

 apparatus would be very small unless a relay were 

 employed, and this would at once put a limit on 

 the speed of working, which wouid seriously 

 hamper its utility. 



P.T. c 



