A. M. Mayer on measuring Electrical Conductivities. 311 
method invented by the illustrious Gauss, using a telescope and 
seale placed as described below 
e telescope was of ‘7 in. aperture and 12 in. focus; just 
under its object glass was placed, at right angles to its axis, 
rod of wood 1 in. square and 1 meter long, cover with 
drawing paper. This rod was divided off into centimeters by 
ines 1™™ thick ; thus, the division lines were ;'; of the distance 
between two similar sides of the centimeter divisions, A thiek 
spider thread was selected which just covered a division line, 
and therefore was also, apparently, 1™™ thick. By this simple 
evice,—using one and the same side of the spider line as point 
Oo cal 
he scale was 2°285 meters distant from the arin of the mir- 
ror, and therefore a motion of 1 division of the scale ae the 
spider thread corresponded to an angular deflection of 7’ 30”, 
and as we have seen that ;, of a division can be sseedicchd 
read, it follows that we can determine a deflection of 45”. In 
this paper I will give the deflections in 
divisions of the scale, which can be con- 
verted into minutes of are by multiply- 
ing them by 
Connecting- block is the name I give to 
the block of wood, placed under the pro- 
jecting end of the magnet; it ~ four 
enna containing mercu eans 
of which we make the various Blecttioa) 
connections required in the experiments. 
2 gives a view of this block and 
shows the manner of ma the con- 
nections when the object is the measure- 
ment of relative electrical resistances. 
, A’ are 
the terminal wires ae one spiral, B; BF 
those of the other. The wires to be 
compared are at E and F. If E repre- 
sent the standard wire of a fixed length, 
then the wire F has to allow of its length 
ing altered so that its resistance may 
be made equal to thatof E. This is pa 
ranged by sliding one end of this 
through a heavy copper clamp (not owe in the Page which is 
fixed in the men ne B, while tie other end, previously 
well amalgamated, dips into eup A’. 
