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ANNUAL REPORT SMITHSONTAN INSTITUTION, 1913. 



as extending out radially from the wire and bending over to meet a 

 considerable portion of the ground surface below. As this arrange- 

 ment is constituted, there is no energy transfer and the condition is 

 static only. If now the far end of the line is earthed, as through an 

 instrimient or device which uses energy, as in figure 5, at the moment 

 of such connection there would be a lowering of the intensity of the 

 stress toward the receiving instrument and the line would be dis- 

 charged were it not for the maintaining action of the battery, which 

 still keeps up the difference of potential between line and ground. If 

 the line is without resistance, this potential will have the same value 

 all along the line, especially if the line is of uniform section and of 

 uniform distance from the ground. The moment, however, the in- 

 strument at " I " takes energy from the line a current is found in 

 the wire and a return in earth, and there is, so to speak, a flow of 



energy in the space between the 

 wire and earth and in the ether 

 surrounding the wire, in the direc- 

 tion of the arrow — that is, from 

 the generating end to the receiv- 

 ing end. Surrounding the wire at 

 this time there will be a magnetic 

 field, which may be represented 

 by whorls or lines of magnetism, 

 so called, wrapped around the 

 wire like so many hoops of all 

 sizes (fig. 6), expanding in size 

 aw^ay from the wire in all direc- 

 tions; and a similar magnetic 

 effect, of course, is also produced 

 by the return current in the 

 earth. But on account of the 

 conditions of conduction in earth being very devious and irregu- 

 lar, it would be difficult to map the magnetism generated. The sys- 

 tem of magnetic whorls so developed on the fl^ow of the current in 

 the system reaches, for any definite current, a definite density after 

 a short interval. In other words, the density of the magnetic field 

 between the wire and the earth increases only up to a certain point. 

 If the current, however, be doubled in any way, that field is doubled 

 in density or there are twdce as many lines packed in the space around 

 the wire. If now we took instead of an earth-connected circuit one 

 in which there are two wires extending from the generating battery 

 or generator, the conditions will be the same except that the stress 

 lines will now radiate from each wire and connect the Avires by lines 

 directly between them and by other curved lines outside. Such lines, 

 or otherwise conceived " tubes of force," represent the static field or 



