42 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
It will be seen that the whole arrangement of wires, battery and galvanometer, 
forms, with the resistance coils a and /3, a “ Wheatstone’s balance,” such that no 
part of the wires to be compared, namely, S and 0, is out of the air-chamber. The 
galvanometer was provided with two sets of needles, which were astatic; the coil 
surrounding the upper set having a resistance of 7’487 ohms, and that surrounding the 
lower set a resistance of 4863 ohms, at the temperature 21° C. The wire of the former 
coil was divided into two equal parts, the ends of these being soldered to four terminal 
screws, so that, by connecting the two parts of the coil in “Multiple arc,” the resis¬ 
tance could, if necessary, be diminished to 1*872 ohms, and the instrument rendered 
available for experiments requiring a galvanometer of low resistance. The upper coil 
only was used in this part of the enquiry, and yet the instrument proved so sensitive 
that, with the single cell employed for the current-motor, and with the adjusting 
magnet assisting the earth’s directive force on the needles, a variation of conductivity 
of 1 part in 100,000”' could be readily detected even in the most unfavourable case, 
which was that of a copper wire having a resistance of only *0224 ohm ; whilst in the 
majority of instances resistances were measured within Jive parts in one million. 
The resistance coils were of platinum-silver, and the wire N N uniting a and f3 
was, in the first few experiments, made of the same material, but was afterwards 
replaced by one of platinum-iridium ; this latter wire was made expressly for me by 
Messrs. Johnson and Matthey, and seemed a marvel of accurate wire-drawing, as on 
testing it at various parts no variation of conductivity could be detected anywhere 
except at the two extreme ends, where it was firmly clamped into brass blocks ; all the 
graduated portion of the wire, 470 millims. in length, seemed, within the limits of 
observation, to be perfectly uniform. 
When required for measuring resistances in the ordinary way, the coils were 
arranged as in fig. 8 ; where X represents the wire under examination, G the 
galvanometer, B the battery, H the sliding-piece, and x, y, z conical plugs of brass 
with ebonite heads. Immediately to the right of X are a set of resistance-coils of 
the “ dial pattern,” ranging from -nyth of an ohm to 10,000 ohms ; whilst below these 
are two sets of resistances, the arrangement and magnitude of which are sufficiently 
shown in the diagram ; the plugs x, x serve to commute the position of X in the bridge, 
and the plug y to throw out, if necessary, the resistance N N, the dotted lines 
representing wires of extremely small resistance. 
* These beautiful instruments might, if necessary, be rendered even much more sensitive than they are 
by adopting a finer suspension. I have found that a single fibre of unspun silk is quite sufficient to sustain 
a weight equal to that on the suspension of many of the galvanometers which are now made, and anyone 
who will take the trouble to test practically, as I have done, the alteration which can be effected in the 
instrument by the careful selection of such a fibre will be surprised at the result. I feel little hesitation in 
asserting that my own galvanometer could be rendered three times as sensitive as it is now by such means ; 
as it is, the silk suspension has been slowly untwisting for upwards of seven years, and still continues to do 
so. I have also found that keeping the galvanometer perfectly stationary, after it has once been placed 
in position, materially assists in preserving the magnetism of the needles. 
