166 Prof. J. C. Bose. On a Self-recovering Coherer and the 



" On a Self -recovering Coherer and the Study of the Cohering 

 Action of different Metals." By Jagadis Chunder Bose, 

 M.A., D.Sc., Professor of Physical Science, Presidency College, 

 Calcutta. Communicated by Lord Eayleigh, F.B.S. Be- 

 ceived March 6,— Eead April 27, 1899. 



In working with coherers, made of iron or steel, some special diffi- 

 culties are encountered in the warm and damp climate of Bengal. The 

 surface of the metals soon gets oxidised, and this is attended with 

 variation of sensitiveness of coherer. The sensitiveness, it is true, 

 does not altogether disappear, but it undergoes a considerable diminu- 

 tion. The presence of excessive moisture in the atmosphere introduces 

 another difficulty. Substances to be experimented on become more or 

 less opaque by absorption of water vapour. As fairly dry weather 

 lasts in Bengal only for a few weeks in winter, the difficulties alluded 

 to above are for the greater part of the year serious drawbacks in 

 carrying out delicate experiments. To avoid as far as possible the 

 partial loss of sensibility of the receiver due to oxidation, I tried to 

 use metals less oxidisable than iron for the construction of the coherer. 

 In my earlier experiments I derived considerable advantage by coating 

 the steel spirals with deposits of various metals. Finding that the 

 sensitiveness depends on the coating metal and not on the substratum, 

 I used in my later experiments fine silver threads wound in narrow 

 spirals. They were then coated with cobalt in an electrolytic bath. 

 The coating of cobalt was at first apt to strip off, but with a suitable 

 modification of the electrolyte and a proper adjustment of the current, 

 a deposit was obtained which was very coherent. The contact surface 

 of cobalt was found to be highly sensitive to electric radiation, and the 

 surface is not liable to such chemical changes as are experienced in the 

 case of steel. 



I next proceeded to make a systematic study of the action of dif- 

 ferent metals as regards their cohering properties. In a previous 

 paper* I enumerated the conditions which are favourable for making 

 the coherer sensitive to electric radiation. These are the proper 

 adjustment of the E.M.F. and pressure of contact suitable for each 

 particular receiver. The E.M.F. is adjusted by a potentiometer slide. 

 For very delicate adjustments of pressure I used in some of the fol- 

 lowing experiments an U-tube filled with mercury, with a plunger in 

 one of the limbs ; various substances were adjusted to touch barely the 

 mercury in the other limb. A thin rod, acting as a plunger, was made 

 to dip to a more or less extent in the mercury by a slide arrangement. 

 In this way the mercury displaced was made to make contact with the 



# "On Polarisation of Electric Ray," 'Journal of Asiatic Society of Bengal,' 

 May, 1895. 



