174 ANNUAL OF SCIENTIFIC DISCOVERY. 



the charge breaks away from its proper channel. There is gen- 

 erally no enlargement of the rod or extension of sm'face where it 

 enters the ground, but merely a continuation of tlie small rod used 

 on the building, thus presenting a very limited surface to the im- 

 perfect contact of imperfect conducting materials, which must im- 

 pose great resistance to the free exit of the charge. The effect of 

 resistance would be more clearly brought out when it was consid- 

 ered that water had sometimes failed to dissipate the charge as 

 freely as it was conveyed from the cloud to the rod. In some cases 

 the glass insulators are broken, and sometimes large quantities 

 of earth are thrown up where the rod enters the ground, and gen- 

 erally a lateral discharge injures the building to the hazard of 

 life. 



If every obstruction to a free escape of the charge to the earth 

 could be removed, he believed that casualties from lightning 

 would be comparatively rare, even if the rods in general use 

 should be left in other respects as they now are. 



INCREASE OF THE QUANTITY OP ELECTRICITY FROM INDUCTION 



COILS. 



In volume xv. of the "Proceedings of the Royal Society," Rev. 

 T. R. Robinson treats of the means by which an increase in the 

 quantity of electricity may be obtained from induction coils. To 

 increasing the power of the exciting batter}^ there are the objec- 

 tions of injury to the acting surface of the contact breaker, and 

 disproportionate minuteness of effect beyond moderate limits ; if 

 we make the outer helix of larger or thicker wire, we find that 

 length gives no increase in quantity, while that obtained by thick- 

 ness soon reaches a limit. The best method is to combine several 

 coils collaterally, in a manner analogous to that in which galvanic 

 cells are united for quantity. 



He arrives at the following general results: 1. Two helices in 

 series give no increase of quantity, though their intensity nearh'' 

 equals the sum. 2. The quantity of helices connected collaterally 

 equals the sum of their separate effects. 3. The quantity in- 

 creases with the diameter of the wire, to a maximum reached 

 when this is about one-sixty-fifth of an inch. 4. Helices on differ- 

 ent primaries combined produce more effect than when on the 

 same primar}*. 



ELECTRICITY WILL NOT PASS IN AN ABSOLUTE VACUUM. 



It is now well established that the electric spark will not pass 

 through an absolute vacuum. M. Gassiot made a vacuum in his 

 apparatus by filling it with carbonic acid and exhausting by the 

 ordinary process, the residue being then absorbed by caustic pot- 

 ash. By the new process of the Alvergniat Brothers, in which a 

 vacuum is produced by means of a mercurial air-pump of their 

 contrivance, this result is shown in an easier and more rapid man- 



