Angle indicators have essentially three parts: a _transmitter_, a _receiver_, and switches. The transmitter (Fig. 10) can sense the exact amount that a shaft has turned and give out a voltage in each of two wires which tells exactly how much the shaft has turned (Fig. 11). The receiving device (Fig. 12), which has a motor, can take in the voltages in the two wires and drive a second shaft, making it turn in step with the first shaft. By means of the switchboard (Fig. 13), the two wires from the transmitter of any angle-indicator can lead anywhere in the machine and be connected to the receiver of any other angle indicator. [Illustration: FIG. 10. Scheme of angle-indicator transmitter.] [Illustration: FIG. 11. Indication of angle.] [Illustration: FIG. 12. Scheme of angle-indicator receiver.] In a differential analyzer, we can connect the shafts together in many different ways. For example, suppose that we want one shaft _b_ to turn twice as much as another shaft _a_. For this to happen we must have a mechanism that will connect shaft _a_ to shaft _b_ and make shaft _b_ turn twice as much as shaft _a_. We can draw the scheme of this mechanism in Fig. 14: a box, standing for any kind of simple or complicated mechanism; a line going into it, standing for input of the quantity _a_; a line going out of it, standing for output of the quantity _b_; and a statement saying that _b_ equals 2_a_. [Illustration: FIG. 13. Switchboard.] One mechanism that will make shaft _b_ turn twice as much as shaft _a_ is a _pair of gears_ such that: (1) they mesh together and (2) the gear on shaft _a_ has twice as many teeth as the gear on shaft _b_ (Fig. 15). On the mechanical differential analyzer that MIT finished in 1930, a pair of gears was the mechanism actually used for doubling. To make one shaft turn twice as much as another by this device, we would: go over to the machine with a screwdriver; pick out from a box two gears, one with twice as many teeth as the other; slide them onto the shafts that are to be connected; make the gears mesh together; and screw them tight on their shafts. [Illustration: FIG. 14. Scheme of a doubling mechanism.] [Illustration: FIG. 15. Example of a doubling mechanism.] On the MIT differential analyzer No. 2, however, we are better off. A much more convenient device for doubling is used. We make use of: a _gearbox_ in whichthere are two shafts that may be geared so that one turns twice as much as the other, and two angle-indicator transmitters and receivers. Looking at the drawing (Fig. 16), we can see that: shaft _a_ drives shaft _c_ to turn in step, shaft _c_ drives shaft _d_ to turn twice as much, and shaft _d_ drives shaft _b_ to turn in step. Here we can accomplish doubling by closing the pairs of switches that connect to the gearbox shafts. [Illustration: Angle indicators: T, transmitters, and R, receivers FIG. 16. Another example of a doubling mechanism.]