50 President's Address. [Feb. 



the difficulty of placing the telegraph line at the disposal of the observers, 

 it will be obvious how much the introduction of the method of Duplex 

 Telegraphy, would facilitate such operations and how important it is that 

 such facilities should exist. 



Another great improvement in the application of electricity is repre- 

 sented by the magneto-electric machine of Gramme, which is based on 

 the principle of Siemens, Wheatstone and Wilde, to accumulate electricity 

 by the transformation of mechanical force into magnetism and electricity. 

 The new features in this machine are the better coiling of the revolving 

 wire and an improved construction of the commutators by which the extra 

 currents are partly eliminated and the total effect of the machine greatly 

 increased. 



The eff'ect of this machine was tried in London last summer for pro- 

 ducing marine signal lights from the top of the Houses of Parliament. 

 The best optical instruments aided the trial and the success was very great 

 and the light brilliant. 



This cheap mode of producing enormous quantities of electricity has 

 further suggested more extended applications of the electric current in other 

 engineering branches and it is not improbable that metallurgy will receive 

 much aid from it in the production of pure metals. 



Before leaving this branch I would ask your attention to the question 

 of Earth Currents. The subject is referred to by Mr. Schwendler in the second 

 part of his instructions for testing telegraph lines. 



Earth Currents. These are caused by a difference of potentials be- 

 tween the two points of the earth with which the earth plates are in contact. 

 To measure these currents and to determine their directions and the electro- 

 motive force producing them, is of the greatest scientific interest. 



The phenomenon of earth currents is generally, however, much obscur- 

 ed by various causes, especially the polarization of earth plates, and by 

 other causes of which we shall speak hereafter. All that we can do is to 

 measure the total effect of all the causes acting, and only rarel}'- are we able 

 to attribute to each cause its proper effect. Still we are able to say from 

 four years' experience of testing Indian lines, that real earth currents do 

 permanently exist, although we have not been able to estimate the electro- 

 motive force or to determine the law of change in direction in each 

 particular case. 



To be in a position to do this, special observations would be necessary. 



Further : Earth Currents attain their maximum strength when those 

 " magnetic storms," occur which seem to accompan}^ all great perturbations 

 of the sun's surface, and are generally marked by the appearance of vivid 

 Auroras. During such periods, as in the autumn of 1819-50 and 1870, com- 

 munication is rendered impossible except in cases when the earth can be 

 thrown off" and a second wire used for the return circuit. 



