4G2 
PROFESSOR W. WATSON ON THE EARTH’S ^lAGNETIC FIELD. 
earth’s magnetism is higher than the value measured in international units hy 
0'00002 C.G.S. unit. 
If the true value of the electro-chemical equivalent of a fresh solution of silver 
nitrate is 0'0011183, as found by Kahle, then the dilference between the galvano¬ 
meter value for H and the Observatory value becomes — O'OOOOI C.G.S. unit ; in 
this case the field, as measured by the coil, is exjmessed in C.G.S. units, and the 
number is derived from the silver depositions only. 
Since the observations with the galvanometer were made in the new magnet-house, 
while the absolute measurements with the Observatory instruments are made in the 
old magnet-house, which is at a distance of about 35 yards from the new one, any 
difference in the value of H inside the two houses will appear as a difference between 
tlie two methods. With regard to this the Director of the National Physical 
Laboratory writes :—“ Such observations as we have do not show any evidence 
of a systematic difference in the value of H in the two houses. They are not, how¬ 
ever, accurate to less than 2y or 3y (2 or 3 in the fifth place) ; and the point is one 
which we intend to investigate more fully. With regard to the difference in the 
values of H as given by the galvanometer method and magnetometer method 
respectively, I should like to point out that Dr. Chree in some recent papers has 
called attention to one or two sources of small error or uncertainty in the magneto¬ 
meter method which may possibly go some way to account for the difference. He 
hopes to investigate this point shortly ; when this has been done it would be 
desirable to have some further comparisons between the two methods, more especially 
as the temperature conditions in the early part of February were not well suited for 
the work.” 
In conclusion, the author wishes to olfer his thanks to Dr. Glazebrook and 
Dr. Chree for their kind assistance while he was making the observations at Kew. 
He is also very much indebted to his colleague, Mr. F. E. Smith, who gave him 
invaluable assistance in making tlie observations at Kew, as well as in the silver 
depositions and the preparation of the standard cells. His thanks are also due to 
the Government Grant Committee, who defrayed the cost of constructing the 
instruments used in the investigation. 
