Context: Manifold vacuum can be used to drive accessories on automobiles. The best-known application is the vacuum servo, used to provide power assistance for the brakes. Obsolete applications include vacuum-driven windscreen wipers and Autovac fuel pumps. Some aircraft instruments (Attitude Indicator (AI) and the Heading Indicator (HI)) are typically vacuum-powered, as protection against loss of all (electrically powered) instruments, since early aircraft often did not have electrical systems, and since there are two readily available sources of vacuum on a moving aircraft—the engine and an external venturi. Vacuum induction melting uses electromagnetic induction within a vacuum.

Question: What provides power assistance for auto brakes?
Answer: vacuum servo

Question: What are two available sources of vacuum on a moving airplane?
Answer: engine and an external venturi

Question: Why are the Attitude indicator and heading indicator vacuum-powered?
Answer: protection against loss of all (electrically powered) instruments,

Question: What does a manifold vacuum do on a car?
Answer: drive accessories

Question: What no longer used accessories were powered by vacuum?
Answer: vacuum-driven windscreen wipers and Autovac fuel pumps

Question: What do electrical systems use within a vacuum?
Answer: Unanswerable

Question: What can electrical systems be used to drive on automobiles?
Answer: Unanswerable

Question: What are electrical systems used to provide power assistance for on a car?
Answer: Unanswerable

Question: What two instruments do electrical systems usually power on some aircraft?
Answer: Unanswerable

Question: Why are the AI and the HI usually powered by autovac fuel pumps?
Answer: Unanswerable