On the Diurnal Variations of the Magnetic Needle. 34-5 



usual degree of disturbance during the presence of aurorae 

 boreales. 



As these effects could not be attributed to the transit of 

 ordinary atmospheric electricity along the wires to the earth, 

 I determined to make a set of experiments on the subject. 



Having obtained delicate galvanometers, I first ascertained 

 that currents are at all times perceptible in the telegraph 

 wires to a greater or less extent when the galvanometer is 

 applied on a sufficient length of wire, and between two earth 

 connections ; but that wires having no earth connexion, or 

 only one, exhibited no currents. 



I also found by simultaneous observations on two galva- 

 nometers, applied one at each extremity of a wire forty-one 

 miles long, that the changes of force and direction of the cur- 

 rents were simultaneous at both ends; the current piissing 

 direct from one earth connexion to the other. 



But the most interesting fact which appeared during these 

 observations, and that which bears immediately on the remarks 

 contained in the letter of M. de la Rive, is that there is a daily 

 movement of the galvanometer needle, similar to that of the 

 horizontal magnetic needle, produced by the electric currents 

 travelling in one direction from about 8 a.m. to 8 p.m., and 

 returning in the opposite direction during the remainder of 

 the twenty-four hours. The times of zero are not regularly 

 maintained, and vary from 7 to 10 o'clock both in the morning 

 and evening; but the greatest regularity is observable in the 

 morning, and the mean result of numerous observations is as 

 above stated. 



This regular diurnal movement of the galvanometer needle 

 is subject to disturbances of greater or less force and duration, 

 which are found to be of greatest energy during magnetic 

 storms, and when aurora is visible; and in these cases the 

 currents are so strong as to affect the ordinary telegraph in- 

 struments, and sometimes prevent altogether the transmission 

 of messages. 



The next experiments were made with a view to ascertain 

 the direction in which these currents alternate ; and the result, 

 as determined from numerous observations, denotes it to be 

 from north-east to south-west. The nearer this line is ap- 

 proached, the more decided is the effect on the galvanometer ; 

 but between east and south, and between north and west, the 

 effect is smaller ; and in approaching north-west and south- 

 east, it becomes indefinite and irregular, but never ceases 

 entirely. 



It also appeared that the effect depended, not on the direc- 

 tion of the wire itself, but on the relative directions of the two 



