Let us take a look at a circuit that will transfer the number (see Fig. 5). First we see two relays in this circuit. They belong to the _C_5 register. The _C_5-2 relay is energized since it holds 1; current is flowing through its coil, the iron core becomes a magnet, and the contact above it is pulled down. The _C_5-1 relay is not energized since it holds 0; its contact is not pulled down. The next thing we see is two _rectifiers_. The sign for these is a triangle. These are some modern electrical equipment that allow electrical current to flow in only one direction. In the diagram, the direction is shown by the pointing of the triangle along the wire. Rectifiers are needed to prevent undesired circuits. Next, we see the bus, consisting of two wires. One carries the impulses for left-hand or 2 relays, and the other carries impulses for the right-hand or 1 relays. Next, we see two relays, called the _entrance relays_ for the _O_ register. Current from Source 1 may flow to these relays, energize them, and close their contacts. When the first line of the program tape is read, specifying the receiving register, the code 1111 causes Source 1 to be energized. This fact is shown schematically by the arrow running from the program tape code 1111 to Source 1. Finally, we see the coils of the two relays for the Output or _O_ register. We thus see that we have a circuit from the contacts of the _C_5 register through the bus to the coils of the _O_ register. [Illustration: FIG. 5. Transfer circuit.] We are now ready to transfer information when the second line of the program tape is read. This line holds 1110 and designates _C_5 as the sending register and causes Source 2 to be energized. This fact is shown schematically by the arrow running from the second line of the program tape to Source 2. When the second line is read, current flows: 1. From Source 2. 2. Through the contacts of the _C_5 register if closed. 3. Through the rectifiers. 4. Through the bus. 5. Through the entrance relay contacts of the _O_ register. 6. Through the coils of the _O_ register relays, energizing such of them as match with the _C_5 closed contacts; and finally 7. Into the ground. Thus relay _O_-2 is energized; it receives current because contact _C_5-2 is closed. And relay _O_-1 is not energized; it receives no current since contact _C_5-1 is open. So we have actually transferred information from the _C_5 register to the _O_ register. The same process in principle applies to all transfers: The pattern of electrical impulses, formed by the positioning of one register, is produced in the positioning of another register. SIMON’S COMPUTING AND REASONING