The most common form of nine-volt battery is commonly called the transistor battery, introduced for the early transistor radios. This is a rectangular prism shape with rounded edges and a polarized snap connector at the top. This type is commonly used in pocket radios, smoke detectors, carbon monoxide alarms, guitar effect units, and radio-controlled vehicle controllers. They are also used as backup power to keep the time in certain electronic clocks. This format is commonly available in primary carbon-zinc and alkaline chemistry, in primary lithium iron disulfide, and in rechargeable form in nickel-cadmium, nickel-metal hydride and lithium-ion. Mercury oxide batteries in this form have not been manufactured in many years due to their mercury content.