_=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? [Illustration: Fig. 92.] _=307. External Resistance; Internal Resistance.=_ When we consider a circuit like that shown in 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.