Context: Most types of capacitor include a dielectric spacer, which increases their capacitance. These dielectrics are most often insulators. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Variable capacitors with their plates open to the atmosphere were commonly used in radio tuning circuits. Later designs use polymer foil dielectric between the moving and stationary plates, with no significant air space between them.

Question: What do many capacitors contain that raises the capacitance?
Answer: a dielectric spacer

Question: What sort of dielectric is used in low capacitance devices?
Answer: a vacuum

Question: What sort of capacitors have traditionally been used in radio tuning circuits?
Answer: Variable capacitors

Question: How did the capacitor traditionally used in radio tuning circuits change over time? 
Answer: use polymer foil dielectric between the moving and stationary plates

Question: What did the later designs of capacitors used in radio tuning circuits no longer have?
Answer: no significant air space

Question: What do many capacitors contain that lower the capacitance?
Answer: Unanswerable

Question:  What sort of dielectric is never used in low capacitance devices?
Answer: Unanswerable

Question: What sort of capacitors have not traditionally been used in radio tuning circuits?
Answer: Unanswerable

Question: How did the capacitor traditionally used in radio tuning circuits stay the same over time?
Answer: Unanswerable

Question:  What did the later designs of capacitors used in radio tuning circuits still have?
Answer: Unanswerable