196 A. M. Mayer—Researches in Electro-Magnetism. 
impossibility of obtaining a current so constant as not to pro- 
duce continual motion and vibration of the needle caused me to 
devise the following method, which I have found from expe- 
rience to be both sensitive and precise. 
ows Jor the comparative measures of electro-magnetic forces. 
—On a table 10 ft. long was drawn a center line and divided into 
oh nt an inch. This line was then accurately placed at 
angles a the magnetic meridian. Two helices, which I 
the helices in the line of their axes. The same battery current 
passed through both helices, and in such pian tee that the N. 
pole of each was facing the needle; by reversing the current, 
the S. poles could be opposed to each other 
Both helices were composed of 10 layers of “Linch “extra cov- 
ered” copper wire, wrapped on copper spools of 8% inches long, 
1-82 inches diam., and having flanges at the ends 1:25 inches 
high. These spools, with their flanges, were split in the direc- 
tion of their length by an opening ‘of 7 ; inch. Each layer of 
coils was saturated with a thick solution of shellac in aleohol and 
covered with thick paper, coated with shellac, before the suc- 
ceeding layer was wra ped, 
elix E contains 5575 feet of wire in 696 turns, and on ac- 
cgunt of its better insulation and greater number of turns is 
eg in strength to helix W. Helix W contains 551 feet 
wire in 688 turns. 
i 
w F 
om HH NHN Pantene 
Re | 
GNar; 
he a = iagram shows the arrangement of the 
appara: Helix E to the east of the compass; helix W to the 
