832 Prof. Callan on an Induction Coil of great power 



ness were less than this, the resultant of the cohesive force would 

 be Jess, and the crust would crack at some intermediate point. 



Every physical consideration seems to indicate that the crust 

 must be very thick ; and the only real calculation of its limit- 

 ing thickness on physical principles, viz. that of Mr. Hopkins, 

 should be received. 



The form the surface at present exhibits may be supposed to 

 have arisen from the contraction and expansion of the parts of 

 this thick crust since it first began to form, producing hollows 

 in which seas and oceans have gathered together their waters, 

 and elevations in continents, table-lands, and mountains ; and 

 therefoi'e the variations of the surface, under its present aspect, 

 are not at all regulated or produced by hydrostatical principles. 



J. H. Peatt. 

 Calcutta, Feb. 22, 1859. 



LII. A brief Account of an Induction Coil of great 2)oiver in pro- 

 portion to its length. By the Rev. N. J. Callan, D.D., Pro- 

 fessor of Natural Philosophy in Maynooth College^. 



IN the construction of induction coils, the principal object of 

 some seems to have been to make the coil in such a way, 

 that with a given length of secondary wire the longest sparks 

 may be obtained. It appears to me that it would be better to 

 make induction coils so that, with a given battery, sparks of the 

 greatest length may be produced. The longer the coil is, the 

 greater will be the resistance of the primary wire to the current 

 of the battery, and the greater the number of cells which will be 

 required to overcome that resistance and saturate the core with 

 magnetism. Hence it is a matter of great importance to make 

 coils in such a way that, whilst they are short, they may pro- 

 duce very long sparks. I have endeavoured to do this; and 

 though the primary and secondary coils of the induction coil I 

 have made are very imperfect, I have succeeded tolerably well. 



The primary coil is made of thick copper wire about 140 feet 

 long : it is 10 inches in length. The conducting power of the 

 copper wire was injured by being frequently coiled on electro- 

 magnets, or on cores of induction coils. 



The secondai-y coil consists of three small coils : two of them 

 are If inch long, the third is only 1| inch. Hence the entire 

 length of the secondary coil is 5 inches. It is only half the 

 length of the primary coil, and is therefore not finished. The 

 secondary coil is made of iron wire. No. 34 gauge ; the thick- 

 ness of the wire is about the y^^th of an inch. The wire is 

 covered only partially with cotton thread. Between each two ad- 

 * Communicated by the Author, 



