HOLLOW LIGHTNING CONDUCTOR CRUSHED BY THE DISCHARGE. 131 



Note on a HOLLOW LIGHTNING CONDUCTOR 

 CRUSHED BY the DISCHARGE. 



By J. A. Pollock, Professor of Physics, 



and 



S. H. Barraclough, Lecturer in Mechanical Engineering 



in the University of Sydney. 



[With Plate III.] 



[Read before the Royal Society of N. S. Wales, December 6, 1905.~\ 



The piece of lightning conductor to which this note refers 

 was submitted to us by Mr. G. H. Clark of Sydney. It 

 consists of a tube originally cylindrical, 17*5 cms. long, the 

 outer diameter being 1*8 cms., made of copper 0*1 cm. 

 thick with a lap joint 0*4 cm. wide where the thickness is 

 0*2 cm. The piece was the upper portion of a pipe 135 

 cms. long, connecting the finial with a copper band running 

 down a chimney. The specimen, photographs of which are 

 given in Plate in., is crushed in a symmetrical manner and 

 shews the characteristic appearance of a tube which has 

 collapsed under external pressure. 



The crushing is probably due to the electrodynamic action 

 of the current. On this assumption if the stress at which 

 the tube gave way was known, one could obtain some know- 

 ledge of the current during the discharge. For the tube 

 under consideration, a calculation, particulars of which are 

 given below, shows that the mechanical effect at any 

 moment due to the current measured in amperes is 

 equivalent to an excess of hydrostatic pressure on the out- 

 side of the tube of n atmospheres, where 



I = 22000 A 

 Io being the total current in the conductor at the given 

 moment. 



