An account of the theory and construction of a direct-reading screened linear radium source dose calculator is given. The instrument, which has been developed from a simpler one described in earlier papers, serves as a source having up to 3 components in tandem not necessarily of the same linear activities, and indicates the actual dosage in roentgens per hour on a specially calibrated microammeter. In fact it simulates a radioactive source, the strengths and lengths of whose components may be varied at will, combined with a direct reading meter for exploration of dose rate distribution. The operational procedure required no mathematical skill.