142 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIX.) 
3190. I have found the needles very constant in their strength for days and weeks 
together. By care, the constancy of their state for a day is easily secured, and that 
is all that is required in comparative experiments. Those which I have in use weigh 
with their axis and indicating wire 9 grains ; and when out of the copper coil vibrate 
to and fro once in 26 seconds. 
3191. With this instrument thus examined, I repeated most of the experiments 
with loops formerly described (3133. &c.), with the same results as before. It was 
also ascertained that the equator of a regular bar-magnet was the place at which the 
loop should be arrested, to produce the maximum action ; and that if it came short 
of, or passed beyond that place, the final result was less. Employing a magnet 12 
inches long, when the loop passed 
o 
2*3 inehes over the pole the deflection was .... 5*91 
4*1 inches over the pole the deflection was .... 7*50 
5*1 inches over the pole the deflection was .... 7‘74 
6*1 inches over the pole the deflection was . . . . 8*16 
8*0 inches over the pole the deflection was .... 7'75 
9*0 inches over the pole the deflection was .... 6*50 
If a 
Fia:. 3. 
- 
^ ii. Revolving Rectangles and Rings*. 
3192. The form of moving wire which I have adopted for experiments with the 
magnetic forces of the earth (3177*)5 is either that of a rectangle or a ring, 
wire reetangle (fig. 3) he placed in a plane, perpendicular to the 
dip and then turned onee round the axis a b, the two parts c d 
and e f will twice intersect the lines of magnetic force within the 
area c e df. In the first 180° of revolution the contrary direction 
in which the two parts c d and e /'intersect those lines, will cause 
them to conspire in producing one current, tending to run round the rectangle (161) 
in a given direction ; in the following 180° of revolution they will combine in their 
effect to produce a contrary current; so that if the first current is from by c e and 
ftod again, the second will be from d by f e and c to d. If the rectangle, instead of 
being closed, be open at 6, and the ends there produced be connected with a commu- 
tator, which changes sides when the rectangle comes into the plane perpendicular to 
the dip, i. e. at every half revolution, then these successive currents can be gathered 
up and sent on to the galvanometer to be measured. The parts ce and df of the 
* A friend has pointed out to me that in July 1832, Nohili made experiments with rotating rings or spirals 
subject to the earth’s magnetic influence ; they were subsequent to and consequent upon my own experiments 
upon swinging wires (171, 148.) and revolving globes (160.) of January 1832 ; but he extended the considera- 
tions to the thickness of the wire ; the diameter of the spirals and the number of the spirals dependent upon the 
length of the wire. The results (tabulated) will be found in vol. i. page 244, &c. of the Florence edition of his 
Memoires. — March 1, 1852. 
