The two electromotive forces are in the same proportion as the two observed distances of the pointer _Z_ from 150° on the _A_ side of the instrument. _For measuring the Intensity of a Current._--For this purpose the instrument is simply used as a sine galvanometer. The connections are made as shown at Pl. xxiv., Figs. 3_a_ and 7. The manipulations _a_, _b_, _c_, and _d_ same as in the second case. _e._--Connect one pole of a battery to terminal II. and put the other pole to earth. _f._--Connect the line to terminal IV. The galvanometer is then to be turned in the same direction as the needle is deflected until the needle coincides with the zero point. Whilst this is being done the large scale on the slate disc will move under the pointer _Z_, which must be left stationary; the sine of the angle indicated by _Z_ will thus give the value proportionate to the strength of the current. Should the shunt box be required, it has to be connected with terminals II. and IV. Fig. 4 shows the same connections as Fig. 7, but without the shunt box, and with the battery commutator. Fig. 3_{a} shows diagram of the same connections but with the key _K_, and Fig. 3_{b} the same without the key. _A Shunt._--A "Shunt" is a second path offered to a current traversing a given circuit, or portion of a circuit, so as to diminish the amount of the current flowing through that portion of the circuit. In the diagram shown at Fig. 89 the shunt diminishes the amount of the current flowing along the circuit between _A_ and _B_. If only 1/Nth of the current is to pass along the circuit between _A_ and _B_ (of resistance _R_) then the resistance of the shunt must equal R/(N - 1). By the aid of shunts it is quite possible to make use of very sensitive instruments to measure powerful currents. _Commutators or Switch Plates._--A commutator or switch plate is an apparatus by which the direction of currents may be changed at will, or by which they may be opened or closed. Bertin's commutator, which is represented at Fig. 90, consists of a small base of hard wood on which is an ebonite plate, this by means of the handle _m_ is turned about a central axis between two stops _c_ and _c'_. On the disc are fixed two copper plates, one of which _o_ is always positive, being connected by the axis and by a plate (+) with the binding screw _P_, which receives the positive electrode of the battery; the other copper plate _i_, _e_, bent in the form of a horse-shoe, is connected by friction below the disc with a plate (-), which plate is connected with the negative electrode _N_. On the opposite side of the board are two binding screws _b_, and _b'_, to which are attached two elastic metal plates _r_, and _r'_.