depend upon the conditions under which the machine is required to operate, It most cases, the machine should be given a complete over- hauling and thorough cleaning at least once a year. Suction is the most desirable method of cleaning, and is especially recommended where metallic and abrasive dusts are encountered. If com- pressed air is used to blow out the machine, it must be dry and of moder- ate pressure (about 25 to 30 lb. psi). Even lower pressure is advisable if conducting and abrasive dusts are present, as such dusts are extremely harmful if driven into the insulation. Wherever possible, the air should be so directed as to prevent the least possibility of such damage. GENERAL OVERHAULING Machines should be given a complete overhauling and thorough cleaning at least once a year or oftener if conditions warrant. ‘Smaller machines, whose windings are not sufficiently accessible, should be taken apart for cleaning. Loose dust and dirt can be removed by suction or compressed air as outlined previously. Heavy dirt and grease may be removed by cloths, brusnes, or scrapers, using caution not to damage vulnerable parts of the machine. Carbon tetrachloride is the most efficient and the safest cleaning agent for the removal of grease, oil, and sticky dirt. It is non- inflammable, but sufficient ventilation should be provided to avoid toxic effects. Inflammable liquids such as gasoline are not recommended because of the fire and explosion hazard involved. The most effective method of applying the cleaning liquids is to spray it on with an atomizer, The pressure may be about 80 lb. for insulation in good 3-37