(B) Press lever 2 of C R (§ 235) for an instant only. This allows the current to enter A G at L. Repeat several times until you thoroughly fix in your mind the direction in which the right-hand end of the needle is deflected. Does the needle jump suddenly when the current passes? (C) Press lever 3 for an instant only. Study the result. _=256. Astatic Galvanoscopes.=_ It is evident that in the ordinary current detector (Exp. 104), the pointing power of the needle has to be overcome by the magnetic field about the coil, before the needle can be forced from its N and S line. Very weak currents will not visibly move the needle in ordinary detectors. To make a sensitive instrument we must have strong fields about both the needle and coil, and we must, at the same time, decrease the pointing power of the needle. Both of these things are accomplished by using an _astatic needle_ in connection with a coil containing considerable wire. The uses of the astatic galvanoscope will be studied more fully in later experiments. CHAPTER XV. GALVANIC CELLS AND BATTERIES. =EXPERIMENT 108. To study the effect of dilute sulphuric acid upon carbon and various metals.= _Apparatus._ A piece each of copper and iron wire 4 in., (10 cm.) long; two narrow strips of sheet zinc (No. 60), one being amalgamated (§ 257); a carbon rod (No. 64); a tumbler (No. 65), partly filled with dilute sulphuric acid, (§ 258); mercury (No. 52). =257. To amalgamate= one of the above zinc strips is to coat it with mercury. Remove all jewelry from the hands before proceeding. Wash the zinc with water, and with a cloth remove all dirt from its surface. Amalgamated zinc is very brittle, so lay it flat upon a piece of board or upon a plate, after dipping it for an instant in the dilute acid. Place a small drop or two of mercury upon the strip, and rub the mercury about upon both sides of the zinc with a cloth made wet with the dilute acid. Mercury clings strongly to zinc or tin, so you may use a narrow piece of either as a spoon to carry a small drop from the supply to the zinc. Tap it upon the zinc to dislodge the drops. Do not get on too much mercury, just enough to coat it, or, at least, that part of it that will be under the acid. Be careful not to break the thin zinc when amalgamating it, as it gets very brittle. It should look bright. (See Apparatus Book § 32, 33.) _Note._ Should any mercury get upon copper plates it may be removed by heating them in a flame.