property does entail a corresponding disadvantage. I t implies a lack of sensitiveness, which is geometrically obvious, in the method when the relative course lies fairly near the critical course. W hether this would give rise to false alarms which might tend to discredit the method, is a question which must be decided by practical experience.
Electrical currents, both alternating and direct, retard the growth of bacteria in liquids through which they are passed, and under certain conditions cause complete sterilisation. The cell-contents are coagulated by the heat generated, or by electrolytic effects within or without the cell. There is the further possibility that protoplasm may be disintegrated by the mechanical action of an alternating current upon molecular charges, similar in effect to that of rapid vibration, which is known to check the growth of, and even to kill, bacteria. Whether the retardation of growth is regarded as the result of changes in the cell or liquid, the effect is largely controlled by the relation between their electrical conductivities. When these are the same the current flows as if the cell were not present, otherwise the current-density in the bacteria is greater or less than that in the liquid, according as their conductivity is respectively the greater or less. In order, therefore, to control effectively the bactericidal action of electrical currents in a liquid, the relative conductivities of the liquid and the contained bacteria should be known. A full summary of the industrial applications of electrical currents in organic processes is contained in Lafar’s ‘Technisches Handbuch der Mykologie.’ In most of the cases referred to the currents were weak and the voltage gradients in the liquid low. The special feature of the present work is that the voltage and current were taken to their highest limit, under the condition that the temperature did not exceed 30° C., with the testing cell cooled by immersion in running water. The ‘Comptes Rendus’ of April, 1896, contained a short account of observations by S. Lortet on the orientation of bacteria in water through which an alternating current was passed. It was stated that only living bacteria orientated, an observation which, if it had been confirmed, would have had bearing on the relation between electricity and life. This is, however, not the case, for bacteria which have been boiled for several hours orientate more freely than before. Lortet’s observations would be at once explained if his emulsion had been sterilised by the addition of an ionising liquid, such as perchloride of mercury, the conductivity of which is greater than that of the germs. The fact is that dead or living bacteria orientate equally well.
There is a definite need in the engineering industry for an instrument which will determine accurately the thickness of a metal wall from one side only, and which is portable, robust, independent of external power supply, and easy to operate. The instrument described in the paper fulfils all these conditions. The method is essentially that of comparing the resistance of a metal wall under test with that of a similarly shaped wall of the same material, of which the thickness is known. Current, usually under 10 amp., is supplied by a 6- or 12-volt battery to two current contacts held against the wall and usually spaced 2 or 3 inches apart. The current in the circuit is adjusted by varying the rheostat or the number of cells of the small car-battery employed, until the very small potential drop indicated by the deflexion of a galvanometer connected to two potential contacts reaches a predetermined fixed value. From a calibration curve the thickness of the wall corresponding to the current registered by the ammeter is determined. The contacts are made on the plates whose thickness is to be measured by means of Evershed and Vignoles spring-controlled rotating points that were originally designed for a four-point method of working. The flexible leads from these, sometimes 100 but usually 30 feet long, are rubber-covered, and are connected to the circuit in the box by plugs and sockets. These are removed for transport and carried in a separate box. The instrument was originally designed for measuring the thickness of boiler tubes, and examples are given showing that its accuracy in this respect is within a few thousandths of an inch. It was found in practice that the instrument could be used to measure the thickness of mild steel plates up to 1·25 inches and of iron castings, up to 3 inches, with the same order of accuracy. It is also used for the measurement of engine cylinder walls to detect core shift. Examples of these are given in the paper. One notable case was that of the examination of a bank of superheater tubes in a boiler that had given much trouble due to the use of an unsuitable chain-grate stoker. Thinned tubes were at once detected and on being cut out the thicknesses indicated by the instrument were confirmed. Examples are given of the regular use of the instrument in determining the thickness of boiler and superheater tubes, ships' hull plates, tank wagons, and various iron castings that were accessible from one side only.
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