THE VELOCITY OF ELECTRICITY. 687 



THE VELOCITY OF ELECTRICITY. 



Br GIFFOED LE CLEAR. 



THE determination of the velocity of electricity has been the 

 ambition of many physicists ; yet at present it is generally 

 conceded that the velocity may be anywhere from the fraction of 

 an inch per hour to millions of miles per second. 



By the popular use of the words " current of electricity " we 

 have grown to think of a fluid flowing through a wire, yet we do 

 not know that there is any such fluid, and consequently we can 

 hardly say that it has a velocity. However, the attempt was 

 made, some years ago, to find the velocity of electricity, consider- 

 ing it as a fluid, by finding the time taken for a signal sent from 

 the Harvard Observatory, Cambridge, to reach St. Louis. The 

 distance between the two places was known, and the gentlemen 

 who conducted the experiment easily found what they supposed 

 was the velocity of electricity by dividing the distance by the 

 time. To understand why this velocity is not really the velocity 

 of electricity, as well as to understand the direction in which 

 physical research is now directed, we must consider what we really 

 do know about electricity. 



When the two poles of a battery are connected by a wire we 

 say a current of electricity is flowing through the wire. The 

 evidences of the so-called 

 current are two: in the -'^-'-- - - __.-,,-... 



first place, the wire is "'V/ , 



heated; and in the sec- / _ ^^-^^^^i-u^-'^-^'f^^- 



onii, a magnetic force is '<^^S^^^^^' 



hood of the wire. It is ^''il^^F''^*"^^^'^'^ 



this magnetic force that ' |-^^iKa?^-:^B^'M' 



interests us, and we must |, ' iy ^MBMBBJlil^ '?.- 



get as clear an idea of it ' ^^^^^^^!^*^^^^Jf- ' 



as possible. We find by .. v^^C:^^^ -'^^g^sff^^^j^r^^^^^ " 

 experiment that in the r ^'''^^^k'^^^^^^^^^^m> 



neighborhood of the wire *^'^l^*^^^^^^^^^' 



a compass needle is '^^^^^'- 



turned from its custom- 

 ary north-and-south posi- 

 tion. The force which so turns the needle we call the magnetic 

 force, and the direction in which the north end of the needle is 

 pulled we call the direction of the magnetic force. 



The adjacent figure is from a photograph of iron filings spread 

 over a plate through which a wire is thrust, perpendicular to the 

 plate. A current is passing through the wire whose cross-section 



