EXPERIMENT 119. To test the power of various substances to conduct galvanic electricity.
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.
EXPERIMENT 120. To find the effect of sulphuric acid upon the conductivity of water.