THE ELECTRIC CONDUCTIVITY OF NITRIC ACID. 379 



and for the purpose of better contact between the copper connectors and tags the 

 former were furnished with leaden weights instead of the more troublesome elastic 

 bands generally used. 



THE COMMUTATOR. 



In our earlier experiments a small induction coil was used according to the well- 

 known KOHLRAUSCH method, but owing to the susceptibility of the nitric acid to 

 polarisation, this apparatus was found to be unsatisfactory. A commutator of the 

 disc pattern was therefore substituted, being driven by a small water-motor served 

 through a Kelvin tap to secure a regular flow of water ; the necessary electric 

 current was obtained from a one-quart Daniell cell.* The speed of the commutator 

 disc was determined by a kymograph, a lever arrangement being fitted up so as to 

 give one vibration for each revolution. A number of experiments showed that the 

 revolutions varied between the extreme limits of 16 and 20 per second; as the 

 disc carried nine commutators, there were for each revolution 18 makes and breaks 

 and 9 reversals; this gives per second 288-360 of the former and 144-180 of the 

 latter. In order to test the efficacy of this method, the alternating currents were 

 led directly into nitric acid of various concentrations for one hour. No evidence of 

 polarisation could be detected when the commutating arrangement was switched oft' 

 and the electrodes simultaneously connected up with a very sensitive galvanometer, 

 and, further, no trace of nitrous acid could be detected by the metaphenylene- 

 diamine test. 



The commutator, working-cell, and water-motor were placed in a different wing 

 of the building, the current being brought into the experimental room by leads 

 twisted together. 



GENERAL ARRANGEMENT OF APPARATUS (Diagram I). 



C represents the commutator, L its leads, K a key for either short-circuiting the 

 current (position 1), or connecting up with the bridge and electrolytic cell E 

 (position 3) or breaking connection altogether (position 2). E 3 is a small bifilar coil 

 to the extremities of which the leads of a call telephone, t, were attached, the latter- 

 serving to detect any alteration or failure of the commutator. R^ is a resistance box, 

 the coils of which were adjusted so as to approximately compensate any alteration in 

 the standard resistance, R, and therefore to maintain the current uniform through 

 the whole series of experiments ; B is a switchboard, E 2 a box of auxiliary cells used 

 in connection with CAREY FOSTER'S method, and t l a balancing telephone. The leads 

 Lj and L 2 connecting the switch-board with the bridge consisted of ten strands of 



* The current of this cell, measured by a standard tangent galvanometer, was found to be '22 ampere 

 when flowing through the commutator at rest, but it would probably be rather less when the 

 commutator was rapidly running, owing to a consequent increase of resistance in the contacts. 



3 C 2 



