116 TRANSMISSIVITY IN PASSIVE IRON WIRES 



minutes the speed is still greater and the distance has increased to 10 

 or 15 cm. ; andafter 3 minutes the wave usually travels rapidly (though 

 less so than before the original activation) over the whole length as 

 before. Recovery is thus a gradual process whose course can be 

 traced by measuring the distance which an activation wave travels 

 at successive intervals after repassivation. In the experiments about 

 to be described the relation between the concentration of acid and the 

 rate of recovery has been determined in this manner for solutions of 

 nitric acid of different concentration. 



The procedure has been as follows. Straight lengths of wire 20 

 cm. long are bent at one end into a hook shape (to facilitate handling, 

 which is done with glass hooks and rods) and treated as already de- 

 scribed so as to secure a uniform surface ; they are then passivated by 

 immersion in strong nitric acid (Baker's ''Analyzed," specific gravity 

 1 .42) , where they may remain for an indefinite time before using. The 

 same wires may be used over and over again in different experiments. 

 In each experiment with a given solution several such wires, e.g. ten, 

 are placed side by side in a flat-bottomed rectangular glass dish (23 

 by 14 cm.) containing the acid; when all the wires are in position they 

 are activated simultaneously (or nearly so) by touching at one end 

 with a bar of zinc; an activation wave then sweeps rapidly over each 

 wire, which receives at the same time a coating of the brown oxide. 

 The wires are now again passive, but at first transmit activation im- 

 perfectly, as just described. The distance traveled by the activation 

 wave after a definite time interval of recovery is now determined by 

 touching the ends of the wires with zinc, one by one, in succession 

 from left to right at regular intervals. For instance, in 65 per cent 

 acid the successive wires are activated at ^ minute intervals ; a series 

 is thus obtained showing the distances traveled after periods of recov- 

 ery lasting respectively ^ minute, 1 minute, 1| minutes, 2 minutes, 

 etc. The distances can readily be measured with a millimeter rule; 

 the limit of each freshly activated area is distinctly marked by the 

 dark shade of the more recently deposited oxide; this boundary re- 

 mains visible for some time after the reaction, so that if desired the 

 measurements can be made at leisure at the conclusion of a series of 

 experiments. 



