A discussion is given of the factors involved in the accurate radiation dosimetry of objects bombarded with initially mono-energetic electrons. It is necessary to match the incident electron energy with superficial density in the case of a parallel-sided target in order to attain uniform dose distribution. A preferable procedure is to employ the maximum electron energy available and to use a beam filter of suitable thickness, interposed between the electron source and (in contact with) the target. Such a combination minimizes the maximum overdose given.Another concept described is that of average dose factor, which arises because of the asymmetry of the normal ionization curve.Details of a simply made instrument are given, enabling electron dosimetry calculations to be made by a mechanical analogue of the normal ionization curve. It shows values for optimum electron energy, filter thickness required, and also for average dose factor once the superficial density of the target is known.