It took experimenters nearly 30 years to realize that electrons given off by hot metals could be used to amplify signals.Thomas Edison suspected as early as 1880 that something was travelling through the vacuum inside his light bulbs.In 1904 Ambrose Fleming used this "Edison effect"to make a diode a device with two electrodes that,because it conducted electricity one way only,he called a valve.It could detect weak radio waves by turning their alternating current into direct current that worked an indicator,but could not increase their power.When Lee De Forest added a third electrode,creating the triode,it became possible to convert a weak signal into a stronger one.The triode was the first true amplifier.