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This book deals with the underlying theory and practical aspects of pressure gauges that are at present in general use. Because of the ever-increasing demands to provide a wider range of sophisticated and reliable vacuum equipment a good understanding of these instruments is of vital importance to all workers in the research and industrial sectors. Of the gauges considered only the mechanical types are absolute, in the sense that they measure pressure directly as a force upon a liquid column or a solid surface. Under ideal conditions it is possible to calculate their sensitiv- ities, which are the same for all gases and vapours. The recent developments in the viscous or molecular damping gauges indicate that these may also be considered absolute. Other gauges are indirect in that they involve the measurement of some secondary phenomenon which is pressure-dependent and therefore these gauges can only be used for measurement after calibration against an absolute standard. The radiometer or Knudsen type gauge has been excluded from the text since these are now only of historic interest. Also no mention is made of the integration techniques involving surface changes (such as work function) although these could have application under very special circumstances. The McLeod gauge is dealt with in some detail, for even though this gauge has few practical applications, it is the most sensitive absolute gauge available and has value as a reference standard.