270 APPENDIX 



it ; it is only required that the plus and minus quantities 

 should be earth and cloud respectively. 



In the same way the human body is liable to be in- 

 fluenced not only by being placed in an earth circuit but by 

 induction ; its normal electromotive force of four or five 

 millivolts can only be a plus quantity in favourable 

 circumstances. 



In reviewing the electrical phenomena consequent upon 

 the operation of such a system as the District Railway, we 

 may read for electrified clouds the effect upon the air of 

 alterations of load, while the iron-clad tubes with their far 

 from perfect insulation must be responsible for artificial 

 earth-currents of such potential as to seriously interfere, 

 over a very considerable area, with electro-diagnosis. 



Similarly in tramway lines where direct current is 

 employed the overhead system is likely to affect the air 

 locally, and the conduit system to charge the earth, although 

 the range of inductive interference is not nearly so great as 

 in the case of railways and tubes. 



Quite apart from these artificial disturbances, the 

 hypothesis that in an electrical sense the earth is zero 

 should not be too readily accepted. Prior to important 

 experiment an " earth " should be tested galvanometrically, 

 and although in certain localities the test may be dispensed 

 with in ordinary work, it is a precaution to be recom- 

 mended. 



As a matter of fact the earth is electrically " patchy," 

 the potential and direction of current varying greatly in 

 different parts of the world. Darwin found the neighbour- 

 hood of the Rio Plata to be peculiarly subject to electrical 

 phenomena and was inclined to suspect that thunder- 

 storms were very common near the mouths of great rivers.* 

 On the East African coast the earth-current has remained 

 at about forty volts for many weeks in succession. At that 

 * Journal of Researches. 



