Look again at Fig. 33 of Letter 9 which I am reproducing for convenience. We considered only what would happen in coil _cd_ if a current was started in coil _ab_. Suppose instead of placing the coils as shown in that figure they are placed as in Fig. 108. Because they are at right angles there will be no effect in _cd_ when the current is started in _ab_. Let the current flow steadily through _ab_ and then suddenly turn the coils so that they are again parallel as shown by the dotted positions. We get the same temporary current in _cd_ as we would if we should place the coils parallel and then start the current in _ab_. [Illustration: Fig 108] The other experiment is this: Starting with the coils lined up as in the dotted position of Fig. 108 and the current steadily flowing in _ab_, we suddenly turn them into positions at right angles to each other. There is the same momentary current in _cd_ as if we had left them lined up and had opened the switch in the circuit of _ab_. [Illustration: Fig 109] Now we know that the atomic loops of iron behave in the same general way as do loops of wire which are carrying currents. Let us replace the coil _ab_ by a magnet as shown in Fig. 109. First we start with the magnet at right angles to the coil _cd_. Suddenly we turn it into the dotted position of that figure. There is the same momentary current in _cd_ as if we were still using the coil _ab_ instead of a magnet. If now we turn the magnet back to a position at right angles to _cd_, we observe the opposite direction of current in _cd_. These effects are more noticeable the more rapidly we turn the magnet. The same is true of turning the coil. The experiment of turning the magnet illustrates just what happens in the case of a transformer with, an iron core except that instead of turning the entire magnet the little atomic loops do the turning inside the core. In the secondary of an iron-cored transformer the induced current is the sum of two currents both in the same direction at each instant. One current is caused by the starting or stopping of the current in the primary. The other current is due to the turning of the atomic loops of the iron atoms so that more of them line up with the turns of the primary. These atomic loops, of course, are turned by the current in the primary. There are so many of them, however, that the current due to their turning is usually the more important part of the total current. In all transformers the effect is greater the more rapidly the current changes direction and the atomic loops turn around. For the same size of electron stream in the primary, therefore, there is induced in the secondary a greater e. m. f. the greater is the frequency with which the primary current alternates.