312 A. M. Mayer on measuring Electrical Conductivities. 
The two spirals I are firmly clamped together, so that when 
slid quickly off the magnet 8, they both have the same direc- 
tion of motion. By referring to fig. 2, the directions in which 
flow the currents, thus produced, can be readily followed. 
Taking spiral A A’, the current flows down the terminal A+ 
main at that distance from the end, as the shellac prevents con- 
tact above the amalgamated portion of the wire. rmi- 
nals of the spirals and of the galvanometer coil were formed in 
the same manner. One end of the wire whose resistance was 
to be compared to the standard copper wire, was uncovered and 
well cleaned for some portion of its length, so that it could be 
drawn through the heavy copper clamp until its length equal- 
led in resistance the standard wire. e wires were then re- 
moved and their lengths accurately measured. 
3. Investigation into the actions which take place in the apparatus. 
In the general introductory description given of the method, I 
have, for simplicity of illustration, assumed that when the two 
spirals,—similar as to form, length of wire and resistance,—are 
slid off the magnet, no current would be sent through the gal- 4 
vanometer. But this cannot be, for the hinder spiral is further “a 
on the magnet than the other by ‘06 in. and therefore cuts more 
“lines of magnetic force,” and also, the two spirals traverse 
simultaneously portions of the field differing in magnetic in- 
The following experiments will exhibit the above action. I 
will call the back and front spirals respectively A ae B. 
