The lead-acid battery is still used today in automobiles and other applications where weight is not a big factor. The basic principle has not changed since 1859. In the early 1930s, a gel electrolyte (instead of a liquid) produced by adding silica to a charged cell was used in the LT battery of portable vacuum-tube radios. In the 1970s, "sealed" versions became common (commonly known as a " gel cell " or " SLA "), allowing the battery to be used in different positions without failure or leakage.