305. Resistance. It is harder for a horse to draw a wagon through deep sand than over a smooth pavement. We may say that the sand holds the horse back—that is, it offers a resistance. The electric current does not pass through all sorts of substances with the same ease, and when it succeeds in pushing its way through a circuit of considerable resistance, we cannot expect it to arrive at the end of its journey without being weaker than when it started. Do we expect this of a man or horse? We shall soon see that there is a definite relation between resistance and the strength of the current at the end of its journey.
EXPERIMENT 118. To study the general effect of "resistance" upon a current.
Apparatus. Galvanoscope, G V (No. 58); resistance coil, R C (No. 79) ([§ 310]); two-fluid cell, 2-F C ([§ 281]); 4 wires with connectors ([§ 226]). Arrange as in [Fig. 92]. The current passes as shown by the arrow, and the circuit may be opened and closed at the metal plate, M P, or by using a key in its place. Properly place G V.
306. Directions. (A) Take the reading of G V in degrees, the current passing through the entire length of R C. (See [§ 310].)
(B) Change the end of wire 4 from binding-post R to M, on R C, so that the current will pass through one-half only of R C. Note the reading of G V.
(C) Remove R C entirely and connect wires 3 and 4 by means of a metal plate. Compare the readings of (A), (B) and (C). What do they show?
307. External Resistance; Internal Resistance. When we consider a circuit like that shown in[121] [Fig. 92] we see that it is composed of two parts, and that we have two kinds of resistances. The wires, instruments, etc., make up what is called the external resistance of the circuit; that is, the part that is external to the cell. The liquids in the cell offer a resistance to the current; this is called internal resistance. (See [§ 314].) The strength of the current depends upon the relation between these two resistances, as will be seen by future experiments, as well as upon the E. M. F. of the cell. As liquids are not as good conductors as metals, the internal resistance of cells may be quite high.
308. Unit of Resistance; The Ohm. Whenever anything is to be measured, a standard, or unit, is necessary. The unit of resistance is called the ohm, in honor of Ohm, who made careful investigations upon this subject. A column of mercury having a length of a little over 3 feet has been taken as a unit. (The column taken is 106.3 cm. with a weight of 14.4521 grams; it has a cross-section of about 1 sq. mm., at a temperature of 0°C.) Mercury is a liquid, and has no "grain" to affect the resistance. For the use of students, 9 ft. 9 in. of No. 30 copper wire, or 39 ft. 1 in. of No. 24 copper wire will make a fairly good ohm. We might, of course, take any other length as our standard; the above, however, will give results that are approximately correct. (See[ wire tables] at the end of this book.)