through Rarejied Air. 321 



As might have been predicted, the discharge of the battery, 

 after its passage through air rareiiedto ygtli of an inch pressui'e, 

 produced no deflection of the galvanometer needle. When one 

 end of the galvanometer coil was connected with the conductor 

 of an electric machine by means of a wire and the cylinder of 

 rarefied air, the other end being in connexion with the earth, a 

 considerable deflection of the needle was caused by turning the 

 machine ; no constant change in this deflection, however, was 

 observable when the position of the cylinder in the circuit was 

 changed, a result which will not surprise when the uncertainty 

 of the experiment is considered. 



Experiments on the Induction Apparatus. 



The small induction apparatus, described at the commence- 

 ment, was excited by means of one of DanielPs elements, and 

 the bundle of iron wires half withdrawn from the primary coil. 

 The coil of a delicate galvanometer was introduced into the cir- 

 cuit of the induction current as well as the glass cylinder used 

 with the Leyden battery, containing air rarefied to y^^^ ^^' li^^ 

 of an inch of pressure ; lastly, a commutator was introduced so as 

 to change the direction of the current in the glass cylinder with- 

 out altering anything in the remaining part of the circuit. This 

 position of the commutator was chosen in order to simplify the 

 experiment itself, as well as its description ; if the commutator 

 is applied so as to reverse the direction of the induction current 

 throughout the whole of its circuit, or, indeed, that of the voltaic 

 current in the primary coil, an t><idible change in the play of the 

 tongue connected with the induction apparatus ensues, which 

 has the effect of still further complicating the experiment. Within 

 the cylinder was a brass knob |^ths of an inch thick, which was 

 at first placed at a distance of -^ths of an inch from the platinum 

 surface. When the small surface was the positive electrode, that 

 is to say, when it was connected with what was at first defined 

 to be the positive pole of the induction apparatus, the needle of 

 the galvanometer suffered a normal and maxinnmi deflection of 

 70°; when the knob was the positive electrode, an abnormal 

 deflection of 10°. By a normal or an abnormal deflection is to 

 be understood one which does, or does not, correspond with the 

 direction of the current. On repeating the experiment, a nor- 

 mal deflection of 10'^ was obtained when the knob was positive. 

 The knob was withdrawn to a distance of 1-j-^^ inch from the 

 platinum surface. The knob being negative, 1 obtained a nor- 

 mal deflection of 40° or 50°; but when the knob was positive, a 

 small abnormal deflection, which afterwards changed to a small 

 normal one. When the knob and surface were 2f inches asunder, 

 it was necessary to increase the intensity of the induction current, 



