"Well, what they found, Davis and Barnes, was that if this velocity was made to be the same as that of the alpha particle there was a loss in the number of deflected particles. If there were no electrons, for example, and no magnetic field, all the alpha particles would be collected over here (Y) and they had something of the order of 50 per minute which they counted over here. Now if you put on a magnetic field you could deflect the alpha particles so they go down here (Z). But if they picked up an electron then they would only have half the charge and therefore they would only be deflected half as much and they would not strike the screen.
Now the results that they got, or said they got at that time, were very extraordinary. They found that not only did these electrons combine with the alpha particles when the electron velocity was 590 volts, but also at a series of discrete differences of voltage. When the velocity of the electrons was less or more than that velocity by perfectly discrete amounts, then they could also combine. All the results seemed to show that about 80' of them combined. In other words, there was about an 80' change in the current when the conditions were right. Then they found that the velocity differences had to be exactly the velocities that you can calculate from the Bohr theory. In other words, if the electron coming along here happened to be going with a velocity equal to the velocity that it would have if it was in a Bohr orbit, then it will be captured.
Of course, that makes a difficulty right away be-cause in the Bohr theory when there is an electron coming in from infinity it has to give up half its energy to settle into the Bohr orbit. Since it must conserve energy, it has to radiate out, and it radiates out an amount equal to the energy that it has left in the orbit So, if the electron comes in with an amount of energy equal to the amount you are going to end up with, then you have to radiate an amount of energy equal to twice that, which nobody had any evidence for. So there was a little difficulty which never was quite resolved although there were two or three people including some in Germany who worked up theories to account for how that might be. Sommerfeld, for example, in Germany. He worked up a theory to account for how the electron could be captured if it had a velocity equal to what it was going to have after it settled down into the orbit."
