Arc-Lamp suitable for use with the Duboscq Lantern. 333 



tion of the system of coordinates ; therefore evidently any 

 function whatever of \/a: 2 +y 2 + z 2 fulfils the same condition, 

 and by this condition no other further property of the func- 

 tion F can be disclosed. As, for example, the value of the 

 above-used function e~ h{x2+y2+z ^ 2 is also entirely independent of 

 the special choice of the system of the coordinates, although 

 it does not permit of being reduced to the form /(a?), <j)(y), 



+(*)■ 



XXXVII. On an Arc-Lamp suitable to be used with the 

 Duboscq Lantern. By. Professor Silvanus P. Thompson, 

 D.Sc* 



[Plate III.] 



THE lamp devised by Foucault and Duboscq, and supplied 

 for so many years by the famous house of Duboscq, 

 fails to fulfil the electrical requirements of the modern physi- 

 cal laboratory, though it has rendered excellent service in the 

 past. Yet the lantern and optical adjuncts of the standard 

 pattern of Duboscq are so widely used that it seemed desirable 

 to find some other arc lamp which, while fulfilling the elec- 

 trical requirements of the case, could be used with the 

 Duboscq lantern. 



Before describing the lamp which I have for twelve months 

 employed for this purpose, I propose to state the conditions 

 to be fulfilled, and the reasons why the old Duboscq lamp 

 fails to fulfil them. 



The modern physical laboratory is usually supplied with 

 electric energy under one of two alternative conditions, namely 

 either at constant potential or with constant current ; more 

 usually under the former condition. If supplied from a 

 dynamo the dynamo may be either series-wound, shunt- 

 wound, or compound-wound. If supplied from accumulators 

 the accumulators will work at constant potential, and will 

 have a very small internal resistance. 



The arc-lamp for laboratory use must be capable of working 

 under the given conditions. No donbt the Duboscq lamp 

 worked fairly when supplied with current from 50 Grove's 

 cells. But in a laboratory where there is another and better 

 and less wasteful source of supply, 50 Grove's cells are not 

 desirable. Though 40 accumulators have an electromotive 

 force almost exactly equal to that of 50 Grove's cells, the 

 Duboscq lamp does not work well with them unless a resist- 

 ance of several ohms is intercalated in the circuit to represent 



* Communicated by the Physical Society. 



