electric field is indeed vertical. and not horizontal. when we move d to some inter mediate angle, we see that the strongest signal occurs when it is oriented as shown because although G is vertical, it does not produce a field that is simply parallel to itself-it is the projection of the acceleration perpendicular to the line of sight that counts. The signal is weaker at 2 than it is at 1, because of the projection effect 28-4 Interference test what happens by side a fe centimeters apart(Fig. 28-3). The law is that the two sources should add their effects at point I when both of the sources are connected to the same generator and are both moving up and down the same way, so that the total electric field the sum of the two and is twice as strong as it was before Now comes an interesting possibility. Suppose we make the charges in SI and S, both accelerate up and down, but delay the timing of s2 so that they are 180 out of phase, Then the field produced by S, will be in one direction and the se field produced by S2 will be in the opposite direction at any instant, and therefore we should get no effect at point 1. The phase of oscillation is neatly adjustable by means of a pipe which is carrying the signal to S2. By changing the length of this pipe we change the time it takes the signal to arrive at S2 and thus we change the phase of that oscillation. By adjusting this length, we can indeed find a place where VIEW there is no more signal left, in spite of the fact that both S, and S2 are moving! The fact that they are both moving can be checked, because if we cut one out an see the motion of the other. So the two of them together can produce zero if Fig. 28-3. Illustration of interference everything is adjusted correctly Now, it is very interesting to show that the addition of the two fields fact a vector addition. We have just checked it for up and down motion, but let us check two nonparallel directions. First, we restore S1 and S2 to the same phase; that is, they are again moving together. But now we turn S, through 90, as shown in Fig. 28-4. Now we should have at point 1 the sum of two effects, one of which is vertical and the other horizontal. The electric field is the vector sum of these two in-phase signals-they are both strong at the same time and go througl zero together; the total field should be a signal R at 45. If we turn d to get the maximum noise, it should be at about 45, and not vertical. And if we turn it at right angles to that direction, we should get zero, which is easy to measure. Indeed we observe just such behavior Now, how about the retardation? How can we demonstrate that the signal is retarded? We could, with a great deal of equip the time at which character of the combination of sources. it arrives, but there is another, very simple way. Referring again to Fig28-3 suppose that S1 and S2 are in phase. They are both shaking together, and they produce equal electric fields at point 1. But suppose we go to a certain place 2 hich is closer to S2 and farther from Sr. Then, in accordance with the principle that the acceleration should be retarded by an amount equal to r/c, if the retard tions are not equal, the signals are no longer in phase. Thus it should be possible to find a position at which the distances of D from Si and S2 differ by some amount A, in such a manner that there is no net signal. That is, the distance A is to be the distance light goes in one-half an oscillation of the generator. We may go still further, and find a point where the difference is greater by a whole cycle that is to say, the signal from the first antenna reaches point 3 with a delay in time that is greater than that of the second antenna by just the length of time it takes for the electric current to oscillate once and therefore the two electric fields pro- duced at 3 are in phase again at point 3 the signal is strong again This completes our discussion of the experimental verification of some of the important features of Eq.(28.6). Of course we have not really checked the 1/r variation of the electric field strength or the fact that there is also a magnetic field that goes along with the electric field. To do so would require rather sophis ticated techniques and would hardly add to our understanding at this point. In any case, we have checked those features that are of the greatest importance for our later applications, and we shall come back to study some of the other properties of electromagnetic waves next year