The application of the universal galvanometer will be clear from the diagrams on Pl ii.; instructions, however, for its practical use are added further on, and also tables for use when measuring conducting resistances. As will be seen from diagram, Fig. 1, Pl. xxiv., the proportion between the unknown resistance X, and the artificial resistance _n_ is, when the deflection is read off on the side of the slate disc marked _A_: X : _n_ = 150 + _a_ : 150 - _a_ or, X = ((150 + _a_) / (150 - _a_)) × _n_. but if read off on the _B_ side of the disc-- X = ((150 - _a_) / (150 + _a_)) × _n_. The values of these two fractions, for every half degree, will be found in the columns headed _A_ and _B_ of the table in the Appendix. [Illustration: SIEMEN'S UNIVERSAL GALVANOMETER. PLATE XXIII] [Illustration: SIEMEN'S UNIVERSAL GALVANOMETER. PLATE XXIII^A] [Illustration: SIEMEN'S UNIVERSAL GALVANOMETER. PLATE XXIV] [Illustration: SIEMEN'S UNIVERSAL GALVANOMETER PLATE XXIV^A] _Measuring Electrical Resistances._--For this purpose the instrument is arranged as a Wheatstone's balance. The connections are made as shown at Pl. xxiv., Figs. 1 and 5, where _X_ is the unknown resistance. _a._--The needle _i_ is to be brought to the zero point of the small cardboard scale by turning the galvanometer _G_ round its vertical axis, taking care that the needle moves with perfect freedom. _b._--The pointer or vernier _Z_ is to be brought, by means of the handle _g_, to the zero point of the large scale on the slate disc. _c._--A plug is to be inserted between the terminals marked III. and IV. _d._--The holes 10, 100, and 1000 are, two of them, to be plugged, and one left open, according to the extent of the unknown resistance to be measured; either 10 or 100 must be left open if the resistance is small, and 1000 if it is large. _e._--The two ends of the unknown resistance are to be connected to terminals II. and IV. _f._--The two poles of some galvanic battery are to be connected to terminals I. and V. When the above-mentioned connections have been made, and on depressing the key _K_, the battery current is sent into the combination and deflects the needle, say, to the right-hand or _B_ side of the instrument, the pointer or vernier _Z_ must then be pushed, by means of the handle _g_, to the _B_ side of the instrument. If this is found to increase the deflection of the needle _i_, the pointer _Z_ should be pushed to the other or _A_ side of the instrument beyond the zero point of the large scale until the needle remains stationary when the key _K_ is depressed.