MAGNETIC DISTURBANCES WITH TERRESTRIAL GALVANIC CURRENTS. 467 
and subsequently the coils were reduced. At present, the entire number of turns of the 
wire about the acting needle of each galvanometer is 200, and the length of the acting 
needle is a little more than 1 inch. The needle is suspended by a human hair. A 
stalk below carries a small plane mirror, whose surface is about \ inch by 1 inch. 
With the assistance of cylindrical lenses, a very neat spot of light is thrown upon the 
revolving barrel by the following arrangement. The beam of light from the lamp falls 
upon a fixed vertical cylindrical lens, and is made to converge in the horizontal plane. 
In this state it falls on the plane mirror, converging in the horizontal plane, diverging 
in the vertical plane. It leaves the plane mirror in the same state, only having received 
the change of direction corresponding to the play of the galvanometer-magnet ; and it 
then falls on a fixed horizontal lens, which makes it converge in the vertical plane. 
(This is the plane of a transverse section of the revolving cylinder, which defines the 
direction of the time-abscissa of the photographic curve.) A single lamp gives light for 
the recording apparatus of both wires. 
The revolving barrels, which had been used for many years in conjunction with the 
self-registering magnetometers of the Royal Observatory, were of glass, selected with 
great care, first by Mr. Brooke, afterwards by Mr. Glaisher, from the extensive stock 
of a large manufacturer; and their circularity and cylindricity of form were almost 
perfect. I was desirous, however, of using for the earth-current-apparatus a barrel 
turned in the lathe. After endeavouring in vain to obtain glass so turned, I succeeded 
in procuring a barrel of ebonite, manufactured by Messrs. Silver. The difficulty of 
securing this occasioned nearly a year’s delay in the completion of the apparatus. 
Another year was added by the excavation of the Magnetic Basement in which the 
apparatus was to be placed, and by the caution necessary to be used while the walls were 
damp. Finally, the apparatus was brought into action and the records were made as a 
matter of routine, on 1865, April 1, and from that time the registers are almost perfect ; 
the principal interruption being that produced by the great snowstorm of 1866, January. 
The instrumental elements on which it appeared desirable to maintain a constant check 
are the following: the freedom of the barrel from end-shake, and the uniformity of its 
rotation ; the zero of time on the line of time-abscissa ; and the zero of galvanic measure 
for the ordinates. And the checks are thus made. 
In regard to end-shake of the barrel, a small lamp throws a light through a definite 
hole in the cover of the barrel, which, as the barrel revolves, forms a photographic line 
parallel to the line of time-abscissa, usually called the “ photographic base-line.” 
In regard to the uniformity of rotation, it has been the practice, since self-registering 
apparatus was established, to produce an interruption in the register, by stopping ofl 
the light for a few minutes, three times every day ; the observer registering from the 
clock-dial the times of stopping and readmitting the light. For the test of circularity, 
this has been done on a limited number of days, -six times in the day, at moments 
extending with approximate uniformity of intervals through the twenty-four hours. 
The operation to which I have alluded, even if performed only three times in a day, 
when used in conjunction with pasteboard scales carefully adjusted to exhibit 24 h for 
