Now suppose we put two different kinds of substances close together, that is, make contact between them. How their electrons will behave will depend entirely upon what the atoms are and how they are piled up. Some very curious effects can be obtained. [Illustration: Fig 70] The one which interests us at present is that across the contact points of some combinations of substances it is easier to get a stream of electrons to flow one way than the other. The contact doesn't have the same resistance in the two directions. Usually also the resistance depends upon what voltage we are applying to force the electron stream across the point of contact. The one way to find out is to take the voltage-current characteristic of the combination. To do so we use the same general method as we did for the audion. And when we get through we plot another curve and call it, for example, a "platinum-galena characteristic." Fig. 70 shows the set-up for making the measurements. There is a group of batteries arranged so that we can vary the e. m. f. applied across the contact point of the crystal and platinum. A voltmeter shows the value of this e. m. f. and an ammeter tells the strength of the electron stream. Each time we move the slider we get a new pair of values for volts and amperes. As a matter of fact we don't get amperes or even mil-amperes; we get millionths of an ampere or "microamperes," as we say. We can plot the pairs of values which we measure and make a curve like that of Fig. 71. [Illustration: Fig 71] When the voltage across the contact is reversed, of course, the current reverses. Part of the curve looks something like the lower part of an audion characteristic. [Illustration: Fig 72] Now connect this crystal in a receiving circuit as in Fig. 72. We use an antenna just as we did for the audion and we tune the antenna circuit to the frequency of the incoming signal. The receiving circuit is coupled to the antenna circuit and is tuned to the same frequency. Whatever voltage there may be across the condenser of this circuit is applied to the crystal detector. We haven't put the telephone receiver in the circuit yet. I want to wait until you have seen what the crystal does when an alternating voltage is applied to it. [Illustration: Fig 73] We can draw a familiar form of sketch as in Fig. 73 to show how the current in the crystal varies. You see that there flows through the crystal a current very much like that of Fig. 62a. And you know that such a current is really equivalent to two electron streams, one steady and the other alternating. The crystal detector gives us much the same sort of a current as does the vacuum tube detector of Fig. 54. The current isn't anywhere near as large, however, for it is microamperes instead of mil-amperes.