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Rh railways has already been made by Prussia, Baden and Bavaria, who have agreed on 16⅔. This figure has been arrived at by the consideration that in many cases the possibility of interchanging power between a railway and general supply might be convenient. Machinery for converting frequency can be built and worked most economically if the ratio of conversion is given by whole numbers, such as 1 to 2 or 1 to 3. Since general supply systems are mostly working with a frequency of 50, and the conversion to half that frequency would still leave the frequency a little too high for the attainment of best working conditions in railway propulsion, a converting ratio of 1 to 3 has been adopted by the States above-mentioned. This gives for the railway a frequency of 16⅔. Italy and Switzerland have adopted a standard of 15, but with a latitude of 10 per cent. up or down, so that at the higher figure they come very nearly into line with the German standard, whilst at the lower figure the Swiss railways have the possibility of linking up, by means of frequency transformers, with some existing works for general supply, whose frequency is in some cases as low as 42.

Whatever may be the frequency adopted