1912.] The Electrical Conductivity of Bacteria. 



339 



5. The Sterilisation of Water by Alternating Currents. 



Several observers working with weak current densities have stated that low 

 frequency alternating currents do not affect bacteria,* and there do not 

 appear to be any recorded cases of sterilisation by them other than of 

 d'Arsonval and Charring who found that B. pyocyayieus in a flask of liquid 

 placed in the core of a coil carrying an alternating current lost its chromo- 

 genic power after 20 minutes' exposure to the induced magnetic field. 

 Whether this was owing to the weak eddy currents set up in the liquid or 

 to the action of the field was not determined. It is, however, improbable, 

 in view of the high current densities required to produce marked effects, that 

 the necessarily feeble eddy currents could have been the cause, which must 

 rather be regarded as a direct action of the magnetic field upon the 

 molecular structure of the cell contents of the particular organisms used. 

 Strong alternating magnetic fields do not appear, from a few trials, to have 

 any characteristic effect on bacteria in general. In making comparative 

 experimental trials of the rate of sterilisation in this way, it is very necessary 

 to use bacteria from the same culture, if possible from the same colony, both 

 on account of the variation of the conductivity discussed in the next 

 section, and of a difference found between bacteria from active and poor 

 growths in their rate of sterilisation by electric currents. B. coli communis, 

 many times sub-cultured, was exposed to an alternating current at a frequency 

 of 80 per second and at a constant pressure gradient of 65 volts per centi- 

 metre in common salt solution having a resistance of 50 ohms per centimetre 

 cube, in one of 200 ohms resistivity, and in one of 500 ohms. They were 

 each exposed for three hours. The ratios of the number of colonies per 

 unit area on the control plates to that on the exposed plates were 

 respectively 150, ll'O, and 17"5. The rate of sterilisation in this case of 

 constant voltage therefore falls rapidly as the resistance increases. The ratio 

 of the reduction ratio to the current is ,the same in the first and last of these. 

 The resistance of the bacteria was not measured in every case, but may be 

 taken to be between 35 and 60 ohms per centimetre cube. 



If, instead of having the same voltage gradient in each trial, the current- 

 density is kept constant, results are obtained which show very clearly the 

 greater influence of higher voltages. Exposures were made in 50, 200, 

 and 1400 ohms solutions, with a current-density of 0'3 ampere per square 

 centimetre in each case. The reduction ratios were respectively 9, 13, and 

 26. The bactericidal effect, therefore, does not depend only upon the 



* Zeit (' Journ. Amer. Med. Assoc.,' Nov., 1910) found that weak alternating currents 

 favoured growth. 



t 'Comptes Rendus de la Soc. de Biol.,' 1893, p. 467. 



