HOLLOW LIGHTNING CONDUCTOR CRUSHED BY THE DISCHARGE. 137 



Where a = 0*9 cms. the force in dynes per square centi- 



J 2 



metre will be given by —±- where I is measured in 



ouu 

 amperes. 



If the pressure of an atmosphere is taken as equal to 10 6 

 dynes per sq. cm. a current I measured in amperes will 



produce an effect equivalent to that of an external pressure 



J 2 



of n atmospheres if n = ° or J = 22000 x Vw. 



OUU X 1U 



Mechanical Considerations. 

 It is desired to estimate the pressure at which the tube 

 under discussion collapsed ; such an estimation is difficult 

 to make with any accuracy, because: — 



a. The condition of the tube and the quality of the metal 



are not known. 



b. The temperature of. the metal at the moment of its 



collapse is quite unknown, and there are very few 

 circumstances to guide one in estimating it, and 



c. The theory of the collapse of tubes is imperfect. 



Obviously all that can be done is to obtain approximate 

 upper and lower limits of the pressure conditions at the time. 



a. The material of which the tube was made would 

 probably have a tensile strength of about 30,000 lbs. per 

 square inch at ordinary temperatures, and an elastic limit 

 of approximately 5,500 lbs. per square inch. Copper has no 

 definite "compressive strength" properly so called, but 

 there is a fairly well marked elastic limit in compression 

 at about 3,500 lbs. per square inch. These figures decrease 

 to a marked degree with increase of temperature ; the 

 tensile strength for example being reduced to approximately 

 20,000 lbs. per square inch at a temperature of 600° Pahr. 

 Copper melts at a temperature of about l,930°Fahr.(1054°C.) 

 but experiments on the strength of copper with varying 

 temperatures have not been carried beyond 1,100° Fahr. 1 



1 The Effect of Temperature on the Tensile and Compressive Properties 

 of Copper. — This Journal Vol. xxxi. 



