198 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1925 



induction or electrostatic induction. It has not yet been possible to 

 obtain sparks between points within a completel}' closed metal tank. 

 Under certain conditions small sparks have been obtained by electro- 

 magnetic induction between points in partly inclosed metal struc- 

 tures. Sparks are readily obtained within tanks from a grounded 

 conductor brought in from the outside when it is slightly insulated 

 from the tank by oxide or otherwise. The spark occurs by electro- 

 static induction or when either the wire or tank is struck. This is 

 interesting in that sparks may be obtained from a grounded con- 

 ductor. 



The above experiments have a practical value, because they show 

 the danger of any lead-in conductors to oil tanks or magazines. 

 They also illustrate the importance of having no projections within 

 a tank. For instance, a metal rivet insulated from a tank by oxide 

 might cause a sufficient spark to ignite gases within the tank. 

 Grounded gauge chains could readilj^ cause internal sparks. 



riRES BY LIGHTNING 



It is quite possible to cause fires by lightning by sparks between 

 isolated metal parts even when direct strokes do not occur. Some 

 model bales of cotton were placed under the model cloud. It was 

 found that the cotton could be ignited by sparks electrostatically 

 induced between the metal bonding strips. 



