Electroformed copper and lead bricks protect the cryostats. Small portable cryostats are available and can run off generators or vehicle inverters. Bath cryostats are similar in construction to vacuum flasks filled with liquid helium. Closed-cycle cryostats consist of a chamber through which cold helium vapour is pumped. Typically cryostats are manufactured with two vessels, one inside the other. Cryostats have numerous applications within science, engineering, and medicine.