240 



INDUCTIVE AFFINITY. 



of platinum attached to them within the bottle, which become 

 the terminal plates of the battery when 

 its wires are thrust into the cavities of 

 two small hollow brass cylinders at- 

 tached externally to the platinum wires 

 of the apparatus. The gases escape by 

 a bent glass tube fitted into the cork of 

 the bottle, and may be collected in a 

 graduated jar at a small pneumatic 

 trough. In this instrument steel plates 

 may be substituted for the platinum, 

 provided a solution of carbonate of 

 potash be used instead of dilute sul- 

 phuric acid. 



Galvanometer.' The sensibility of 

 the magnetic needle to the influence of the conducting wire of a 

 voltaic circle brought near it, has been applied to the construc- 

 tion of an instrument which will indicate the feeblest polariza- 

 tion or slightest current in the connecting wire. It consists of 

 a pair of magnetic needles (Fig. 28,) fixed on one axis with their 

 FIG. 28. attracting poles opposite 



each other, so as to leave 

 them little or no direc- 

 tive power and render 

 them astatic, which is 

 delicately suspended by a 

 single fibre of unspun 

 silk. The lower needle is 

 enclosed within a circle 

 formed by a hank of co- 

 vered wire, of which the 

 extremites a and b termi- 

 nate in little cups con- 

 taining mercury. When the terminal wires of a battery are 

 introduced into the same cups, the hank of wire of the gal- 

 vanometer becomes part of the connecting wire, and the needle 

 is deflected. The inductions proceeding in one direction above 

 the needle and returning in the opposite direction below 

 needle, conspire to produce the same deflection ; and the 

 upper needle having its poles reversed, is deflected in the same 



