_Apparatus._ Galvanoscope, G V (No. 58); dry cell, D C, or two-fluid cell, 2-F C; pieces of different metals; wood, dry and damp; tumbler of pure water; rubber; ebonite; silk; glass, etc., etc. _Arrange_ as in Fig. 92, leaving out R C, and instead of having M P between wires 1 and 2, use their free ends to press firmly upon the ends of the substance to be tested; that is, the body under test should take the place of M P in the Fig. G V will show a deflection, of course, when the particular thing under test is a conductor. =311. Directions.= (A) Make tests with the above substances, and with any others at hand, and note which are conductors and which are not. _=312. Conductors and Nonconductors.=_ It is evident, from the experiments, that bodies which conduct static electricity do not necessarily conduct galvanic electricity. The greater the E. M. F. of a current, the greater its power to overcome resistance. Some bodies, like dry wood, that readily conduct the high potential static electricity, make fairly good insulators for the low potential galvanic currents. For convenience, substances may be divided into _good conductors_, _partial conductors_, and _insulators_, or nonconductors. _=Good Conductors.=_ Metals, charcoal, graphite, acids, etc. _=Partial Conductors.=_ Dry wood, paper, cotton, etc. _=Insulators.=_ Oils, porcelain, silk, resin, shellac, ebonite, paraffine, glass, dry air. [Illustration: Fig. 95.] =EXPERIMENT 120. To find the effect of sulphuric acid upon the conductivity of water.= _Apparatus._ Galvanoscope, G V; cell; 2-F C; connecting wires; saucer or tumbler, S; a little sulphuric acid. _Arrange_ as in Fig. 95. =313. Directions.= (A) Put a little pure water in S, and see if enough current can pass through it to deflect the needle of G V. The ends of the wires, 1 and 2, should be gradually moved toward each other, the needle being watched. (B) Put 4 or 5 drops of concentrated acid into the water; stir it, then repeat the test. What effect has the acid? _=314. Internal Resistance.=_ As found in Exp. 120, pure water is not a good conductor of galvanic electricity. The acid in the simple cell, and in other single-fluid cells, acts upon the zinc and at the same time makes it possible for the current to pass, as it reduces the internal resistance. As seen later, this resistance in cells is greatly diminished by bringing the plates near each other, and by increasing the surface of the plates that are in contact with the acid. The larger the plates the less the internal resistance, other things remaining the same. The internal resistance of a _battery_ can be changed by connecting the cells differently. (See Chap. on Arrangement of Cells.) [Illustration: Fig. 96.] =EXPERIMENT 121. To find what effect the length of a wire has upon its electrical resistance.=