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and that the students had themselves benefited greatly in coming 

 under his influence. 



Important as his various contributions to the experimental side of 

 magnetism are, Hopkinson rendered an even greater service to the 

 subject by his definite formulation of the theory of the magnetic 

 circuit. This was contained in the paper on dynamo-electric machinery 

 written in conjunction with his brother Edward,* to which allusion has 

 already been made. The conception that the whole line-integral of the 

 magnetic force is divisible into a series of terms for the substances of 

 various permeability of which the circuit may be composed was as 

 fruitful as it was simple. It threw a flood of light on phenomena 

 which before that had received only empirical treatment. The notion 

 of the magnetic circuit had been vaguely present to the minds of 

 several earlier writers. Hopkinson's expression of it made it for the 

 first time clear and convincing, and the use to which it was put in the 

 same paper demonstrated its value on the practical side, by showing 

 its applicability to dynamo design. 



Dr. Hopkinson was elected a Fellow of the Society in 1878. He 

 served twice on the Council, and in 1890 a Royal Medal was awarded 

 to him for his researches in magnetism and electricity. Speaking on 

 that occasion at the anniversary banquet, in reply to the toast of " The 

 Medallists," he described himself as a professional man desiring to 

 further the pure science of his subject on lines suggested by his pro- 

 fessional work. He owed, he said, to his father his first taste for 

 science, and to Sir William Thomson his first impulse towards 

 research. 



He married in 1873 Evelyn, daughter of Gustave Oldenburg, who 

 survives him with three children. Three others, two daughters and a 

 son, were killed with their father in the accident which brought Dr. 

 Hopkinson's brilliant career to an untimely end. 



A devoted lover of the mountains and an accomplished climber, he 

 generally spent the autumn in the Alps with his family, who shared his 

 taste and, in great measure, his skill. They spent August last at 

 Arolla, and were much on the mountains. On the morning of August 

 27, Dr. Hopkinson set out, with his son Jack and his daughters Alice 

 and Lina, to climb the Petite Dent de Veisivi, a rocky ridge above 

 Evolena, offering no particular difficulty to a party of their experience. 

 When they failed to return at nightfall search parties were organised, 

 and at daybreak, on the 28th, the four bodies were found under the 

 cliff's, roped together, having fallen from a height of some 500 feet. 

 The cause of the accident is not known ; but it is probable that the 

 son, who was leading, slipped in consequence of a portion of the rock 

 giving way, or that he was swept down by a falling stone. It has been 

 well described as the saddest Alpine accident ever known. 

 * ' Phil. Trans.,' 1836. 



