634 



REPORT — 1895. 



•were very small, and those for the later epoch were of opposite signs, according as 

 they were calculated from the earlier or the later survey. 



The same calculation -was made for two other circuits, one (y) in Great Britain 

 and one (S) in Ireland ; but instead of using the calculated terrestrial lines, the 

 true values of the forces and declinations were employed, as deduced from the 

 nearest stations. The great local disturbances in Antrim interfered with the value 

 of the Irish circuit. The following table gives the results in amperes per square 

 kilometre obtained from circuits a and (3 by the terrestrial lines for 1886-0, and 

 also by the mean terre.strial lines for 1891'0, which occupy the mean position 

 between those given by the two independent surveys, and lastly the results for 

 circuit (-y). 



Circuit. 



From these we may conclude that there is not in the United Kingdom a 

 vertical current amounting on the average to O'l ampere per square kilometre. 

 There may possibly be a current of about a tenth or a twentieth of that amount, 

 and if so the signs show that it probably Hows from air to earth, but on account 

 both of the smallness of the results and of the discrepancy between the values 

 obtained for 1891 by two independent calculations, we cannot assert that such a 

 current actually exists. 



The calculations do not disprove Dr. Schmidt's hypothesis, as we cannot argue 

 from the condition of a small portion of the earth's surface to that of the whole. 

 The most that can be said is that no evidence in favour of the existence of vertical 

 currents can be drawn from one district which has been very minutely surveyed. 



On the Equation Connecting the Potential Difference, Current, 

 and Length of the Electric Arc. By Mrs. Ayrton. 



4. On the back E.M.F. and True Resistance of the Electric Arc. 

 By Professor W. E. Ayrton, F.R.S., and T. Mather. 



5. Wote on the Electrolysis of Iron Salts. 

 By W. M. Hicks, F.R.S., and L. T. O'Shea. 



To prepare iron free from sulphur and carbon by electrolysis we used a solution 

 of the double ferrous ammonium chloride. Large masses of metal can only be 

 obtained by paying particular attention to the following points. 



1. The strict neutralitj^ of the solution: its strength must be regulated so 

 as to ofler suitable resistance for the regulation of the potential difference 

 between the terminals and of the current density. We used a five per cent, 

 solution of ferrous chloride to which sufficient ammonium chloride was added to 

 form the double salt. This strength must be maintained, for if the amount of iron 

 salt falls too low the ammonium chloride is decomposed and ferrous hydroxide is 

 precipitated. 



The solution should be free from ferric salt, as this causes the formation of 

 ferric hydroxide, which settles on the cathode plate. The solution can be freed 

 from ferric salts by shaking with reduced iron and filtering just before being 

 used. 



