THE time always comes when some type of machinery reaches the limit of its possible development, and if progress is to continue it must be along quite different lines. The physical properties of the materials at his disposal and the laws of Nature fix limits to the designer's progress in every direction. For example, the reciprocating steam engine, after a century of development, reached its limit with an output of about 10,000 horse-power. Then came Parsons's turbine principle, and the piston engine soon became obsolete in electric power stations. In the Monthly Transactions of the Junior Institution of Engineers of October, Mr. J. Rosen, of Messrs. C. A. Parsons and Co., Ltd., Newcastle, describes a similar change that is taking place in high-voltage production. Sir Charles Parsons and his colleagues in 1921 invented a new type of generator which permitted the generating voltage to be multiplied several times without submitting the insulation to any greater stresses than those in common use in standard machines. Six years ago a turbo-alternator of 25,000 kilowatt power working at a pressure of 34 kilovolts was installed in the Brimsdown Station of the North Metropolitan Electricity Supply Co. and has now been in uninterrupted service for six years. It has been the precursor of many others. The new construction consists of using triple concentric conductors instead of the usual three separate conductors in the stationary armature. The new machines obviate the necessity of using expensive transformers to raise the pressure and thus considerable economies are effected. Mr. Rosen thinks that the new improvements being developed in the composition of insulating materials will lead to the use of much higher voltages in the immediate future.