al resistances, namely
_R_{1}_ and _R_{2}_, and _Z_{1}_ and _Z_{2}_. If we connect a
generator, _G_, between the junctions _a_ and _b_ there will be two
separate streams of electrons, one through the R-side and the other
through the Z-side of the circuit. These streams, of course, will not
be of the same size for the larger stream will flow through the side
which offers the smaller resistance.

[Illustration: Fig 130]

Half the e. m. f. between _a_ and _b_ is used up in sending the
stream half the distance. Half is used between _a_ and the points _c_
and _d_, and the other half between _c_ and _d_ and the other end. It
doesn't make any difference whether we follow the stream from _a_ to
_c_ or from _a_ to _d_, it takes half the e. m. f. to keep this
stream going. Points _c_ and _d_, therefore, are in the same condition
of being "half-way electrically" from _a_ to _b_. The result is that
there can be no current through any wire which we connect between
_c_ and _d_.

Suppose, therefore, that we connect a telephone receiver between
_c_ and _d_. No current flows in it and no sound is emitted by
it. Now suppose the resistance of _Z_{2}_ is that of a telephone
line which stretches from one telephone station to another. Suppose also
that _Z_{1}_ is a telephone line exactly like _Z_{2}_ except
that it doesn't go anywhere at all because it is all shut up in a little
box. We'll call _Z_{1}_ an artificial telephone line. We ought to
call it, as little children would say, a "make-believe" telephone line.
It doesn't fool us but it does fool the electrons for they can't tell
the difference between the real line _Z_{2}_ and the artificial
line _Z_{1}_. We can make a very good artificial line by using a
condenser and a resistance. The condenser introduces something of the
capacity effects which I told you were always present in a circuit
formed by a pair of wires.

[Illustration: Fig 131]

At the other telephone station let us duplicate this apparatus, using
the same real line in both cases. Instead of just any generator of an
alternating e. m. f. let us use a telephone transmitter. We connect the
transmitter through a transformer. The system then looks like that of
Fig. 131. When some one talks at station 1 there is no current through
his receiver because it is connected to _c_ and _d_, while the
e. m. f. of the transmitter is applied to _a_ and _b_. The transmitter
sets up two electron streams between _a_ and _b_, and the stre