A process for producing compounds of general formula useful as the developer for leuco dyes to be used in thermosensible papers, which is excellent in productivity, economical efficiency and safety and permits the production of the compounds from constantly available raw materials. The apparatus further includes a beam generator to irradiate, with a beam, a portion of the sample placed on an electrode with the second polarity. The device further can comprise a solar power module associated with the mobile base unit that converts solar energy into electrical current. The portable DC cooling fan of the invention includes a housing having a plurality of openings on its front and rear surfaces that enable maximum air flow through the device.