Measure of Resistance to Galvanic Currents. 31 
The resistance of these tubes filled with mercury at 0° was 
next compared with one of Jacobi’s standards (B), by means of 
a Wheatstone’s bridge. As the bridge in question, in the form 
used by Halske and myself, is adapted to very accurate measure- 
ments, its more particular description will not be without interest. 
Fig. 3 is a perspective view of the bridge. AA is a brass 
frame on which the slide BB moves. The button C on the 
slide BB is provided with a toothed wheel, which works into a 
toothed rack attached to the frame. The slide-may therefore be 
moved either by direct pressure or by turning this button. 
Attached to the frame are the insulated pieces of metal HH, 
and the graduated scale mm divided into millimetres. Between 
the insulated pieces of metal EE, whose inner surfaces are per- 
pendicular to the scale mm, and are exactly 1000 millims. apart, 
a platinum wire of about 0°16 millim. in diameter is stretched. 
This wire, the ends of which correspond exactly with the division 
marks 0° and 1000 of the scale, is clasped by two small platinum 
rollers, whose axes are connected with the slide B by means of 
the springs G. The bodies whose resistances are to be compared. 
are inserted between the metallic band H, which can be con- 
nected by means of the contact lever I with one pole of the 
battery, and the two thick copper rods, L and L, which move 
freely in the eyes K and K. The other pole of the battery (a 
single-celled Daniell’s battery was generally employed) is con- 
nected with the slide B and the platinum roller. The eyes 
K K, and the insulated pieces EH, which serve as points of 
attachment for the platinum wire, are placed im perfect connexion, 
by means of thick copper rods, with the four plates of the plug 
reverser 8. By changing the plugs, the two resistances to be 
compared could be exchanged. The ends of the multiplying 
wire of the galvanometer employed are connected with the pieces 
of metal HH. In the following measurement I used a mirror 
galvanometer, with a round steel mirror 32 millims. in diameter, 
and 36,000 coils of a copper wire 0°15 muillim. thick. The 
distance of the scale divided into millimetres from the mirror 
was about 64 metres. 
The measurements obtained by means of this apparatus, which 
are collected together in the following Table, were for the most 
part determined by Dr. Esselbach. The method he pursued was 
as follows :— 
Hach end of the glass tube to be tested was inserted in a 
receiver, and retained there by means of a caoutchouc band. 
This receiver was so placed that the unused neck projected up- 
wards, and in this position it was, together with the tube, 
plunged into a trough filled with lumps of ice. One of these 
_ receivers was then supplied with clean dry mercury, which filled 
