976 MESSRS. A. WALLER AND A. DE WATTEVILLE OJST THE IHELUEHCE 
verted into a descending series; increasing excitability ivhen a 'previously descending 
series is converted into an ascending series. 
Tracing 26 shows that it is not indifferent whether make and break of a galvanic 
current he effected in the principal circuit, or in a deriving metallic circuit. 
Electrical connexions were established as in Diagram IV.; they are substantially 
the same as in Diagram II. with the substitution of a battery Tib. for the coil S.c., 
and with the addition of a Helmholtz’ key (H.h) so arranged that the current of 
T.b. can be added to or subtracted from the current of P.b. without breaking circuit 
and without short circuiting T.b. for any appreciable period. By reference to the 
diagram it will be understood that the make and break of T.b. occur at the end a of 
the key; by depressing the handle T.b. is bridged at a, and its current cut off, further 
short circuiting being prevented by the instantly succeeding break at b. By raising 
the handle the bridge is re-established at b, and instantly after broken a,t a , so that 
the current is made. The two inferior surfaces at a and b are connected ; a resistance 
not less than that of the battery T.b. being introduced between the battery and 
the upper surface of the side a of the key, so that the current of P.b. be not transferred 
to a path of less resistance when the testing battery is cut out of circuit. This 
resistance was only used unfavourably to the results anticipated, viz. : in examining for 
increased excitability during polarisation; but its use was found to be a superfluous 
precaution as had been found for the resistance R L . 
We always used the two galvanic currents in the same direction, hence we obtained 
by the above arrangement a comparison of the effects of a given current with the 
effects of the same current added to or subtracted from a pre-established current, viz. : 
Anodic make alone or added to an anodic current. 
Kathodic „ „ a kathodic „ 
Anodic break alone or subtracted from an anodic current. 
Kathodic „ „ a kathodic „ 
The experiments on which the following statements are based, were made with 
11,000 ohms additional resistance in circuit:— 
