On the Lagging of Pipes and Wires. 511 



2. A table is given showing the average values for the 

 surface density of a number of liquids at their boiling- 

 points. 



3. A table is also given showing the variation of the 

 surface density of water with temperature ; the temperature 

 variations of the internal pressure, both in the bulk of the 

 liquid and in the surface-layer, are likewise tabulated. 



4. The heat effect involved in this density change is 

 calculated per unit area of surface and is found to be 0*0034 

 cal. — a quantity which agrees approximately with Parks' 

 analogous determinations. 



I should like to take this opportunity of expressing my 

 indebtedness to Prof. W. B. Morton, the Queen's University, 

 Belfast, and to Prof. A. W.Porter, University College, London. 



LI. On the Lagging of Pipes and Wires. By Alfred W. 

 Porter, B.Sc, Fellow of and Assistant-Professor of 

 Physics in, University College, London ; with an Addendum 

 in conjunction with Mr. E. R. Martin, B.Sc* 



ri^HE effect of surrounding a pipe or wire with a lagging 

 JL material does not seem to be perfectly understood. A 

 coat of poor thermal conductivity may keep a hot pipe cooler 

 than it would be without such a coat. The problem occurred 

 to me a few years ago in w r orking out examples to set to a 

 class of senior students ; but it is also a problem of practical 

 importance, especially in connexion with laboratory apparatus. 

 On looking the question up I find only a reference to such 

 an effect in a paper by Professor Bottomley (Roy. Soc. Proc. 

 vol. xxxvii. 1885), where it is shown experimentally that a 

 difference of a degree or so can exist between two equal wires 

 carrying the same current, one of them coated with a thin 

 coating of various materials such as shellac, while the other 

 is bare ; the coated one being the cooler of the two. Such 

 experiments as these give, however, an entirely inadequate 

 notion of the possible magnitude of the effect as the sequel 

 will showf. A striking experiment is to take a thin platinum 

 wire and coat it at intervals with a thin layer of glass — 

 which is easily fused round the wire. If a suitable current 

 be now passed through the wire the uncoated portions may 



* Communicated by the Author. 



t From a remark in Professor Bottomley 's paper I gather that 

 Lord Kelvin was aware of the anomalous results to be expected. The 

 matter, however, seems to have dropped out of sight owing to the lack 

 of success in obtaining any experimental support. 



