A magnet pulling one way and a spring pulling the other way are sufficient in an ordinary relay to give 2 positions, “on” and “off,” “yes” and “no,” 0 and 1. But how do we make a relay that can hold any one of 10 positions? Figure 4 shows one scheme for a _ten-position relay_. The _arm_ can take any one of 10 positions, connecting the contact _Common_ to any one of the contacts O, 1, 2, 3, 4, 5, 6, 7, 8, and 9 so that current can flow. The _gear_ turns all the time. When an impulse comes in on the _Pickup_ line, the _clutch_ connects the arm to the gear. When an impulse comes in on the _Drop-out_ line, the clutch disconnects the arm from the gear. For example, suppose that the ten-position relay is stopped at contact 2, as shown. Suppose that we now pick up the relay, hold it just long enough to turn 3 steps, and then drop it out. The relay will now rest at contact 5. [Illustration: FIG. 4. Scheme of a ten-position relay, or counter position.] [Illustration: FIG. 5. Scheme of a counter wheel.] In the Harvard machine, the ten-position relays, much like the scheme shown, do the same work as _counter wheels_ (Fig. 5) in an ordinary desk calculating machine, and so they are often spoken of as _counter positions_ in the Harvard machine. They are very useful in the machine not only because they express the 10 decimal digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 but also because adding and subtracting numbers is accomplished by turning them through the proper number of steps. In fact, an additional impulse is provided when the counter position turns from 9 to 0, for purposes of carry. A group of 24 counter positions makes up each _storage counter_—or _storage register_—in the machine. There are 2200 of these counter positions. Each is connected to a continuously running gear on a small shaft (Fig. 6). All these shafts are connected by other gears and shafts to a main drive shaft, and they are driven by a 5-horsepower motor at the back of the machine. When a counter position is supposed to step, a clutch connects the drive to the running gear, and the counter position steps. When the counter position is supposed to stay unchanged, the clutch is disconnected and the driving gear runs free. In fact, when you first approach the Harvard machine, about the first thing you are aware of is the running of these gears and the intermittent whirring and clicking of the counter positions as they step. The machine gives a fine impression of being busy! [Illustration: FIG. 6. Scheme of counter 16.] Timing Contacts A _button_ (see Fig. 7) is a device for closing an electric circuit when and only when you push it. A simple example is the button for ringing a bell: you push the button, a circuit is closed, and something happens. When you let go, the circuit is opened. The Harvard machine has a button for starting, a button for stopping, and many others. [Illustration: FIG. 7. Button.] [Illustration: FIG. 8. Cam, with 5 lobes and contact.]