132 J. A. POLLOCK AND S. H. BARRACLOUGH. 



For reasons given later, we believe that the material of 

 the tube was probably plastic at the time of collapse, owing 

 to the heat developed by the discharge. If this is so, the 

 pressure required to produce the observed folding at any 

 assumed temperature, cannot be calculated ; the theory 

 applicable to such a case and the data of the mechanical 

 properties of copper at the high temperatures here con- 

 templated are alike wanting. Possibly the material was 

 in such a state that the tube gave way under forces 

 equivalent to an excess of pressure outside of the order of 

 not more than an atmosphere ; this would indicate a cur- 

 rent of about 20,000 amperes, neglecting any consideration 

 of the oscillatory character of the discharge. 



On the other hand if the material was not plastic, 

 assuming a temperature as low as 500° C. at the time of 

 crushing we estimate that the collapsing pressure would 

 be of the order of 400 flbs. per sq. inch ; forces equivalent 

 to such a pressure would be produced by a current of about 

 100,000 amperes. 



To illustrate the action of the current suggested above, 

 extremely thin tubes were made by depositing copper 

 electrolytically on silvered glass rods. The ends of the 

 tubes were thickened and the glass rods afterwards removed. 

 On passing a current along the tubes, at a red heat they 

 showed definite signs of collapse though not the charac- 

 teristic folding exhibited by the piece of lightning con- 

 ductor. In the Report of the Lightning Rod Conference 

 (Spoil 1882) on p. 214, a detailed account is given of a 

 hollow conductor which had been struck by lightning, but 

 no mention is made of any appearance of collapse. The 

 tube was however stilfer than the one submitted to us, the 

 external diameter being 1*27 cms. as against 1*8 cm. and 

 the smallest thickness 0*16 cms. as against 0*10 cm. 



