In 1884 the ring of plaster around the top of the base was omitted; in 1886 an improvement was made by pasting the filament to the leading-in wires with a carbon paste instead of the electro-plating method; and in 1888 the length of the base was increased so that it had more threads. Several concerns started making incandescent lamps, the filaments being made by carbonizing various substances. “Parchmentized” thread consisted of ordinary thread passed through sulphuric acid. “Tamadine” was cellulose in the sheet form, punched out in the shape of the filament. Squirted cellulose in the form of a thread was also used. This was made by dissolving absorbent cotton in zinc chloride, the resulting syrup being squirted through a die into alcohol which hardened the thread thus formed. This thread was washed in water, dried in the air and then cut to proper length and carbonized. [Illustration: STANDARD EDISON LAMP, 1884. The ring of plaster around the neck of previous lamps was omitted. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.] [Illustration: STANDARD EDISON LAMP, 1888. The length of the base was increased so it had more threads. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.] The filament was improved by coating it with graphite. One method, adopted about 1888, was to dip it in a hydrocarbon liquid before carbonizing. Another, more generally adopted in 1893 was a process originally invented by Sawyer, one of the Americans who had attempted to “sub-divide the electric light” in 1878–79. This process consisted of passing current through a carbonized filament in an atmosphere of hydrocarbon vapor. The hot filament decomposed the vapor, depositing graphite on the filament. The graphite coated filament improved it so it could operate at 3½ lumens per watt (initial efficiency). Lamps of 20, 24, 32 and 50 candlepower were developed for 110-volt circuits. Lamps in various sizes from 12 to 36 cp were made for use on storage batteries having various numbers of cells and giving a voltage of from 20 to 40 volts. Miniature lamps of from ½ to 2 cp for use on dry batteries of from 2½ to 5½ volts, and 3 to 6 cp on 5½ to 12 volts, were also made. These could also be connected in series on 110 volts for festoons. Very small lamps of ½ cp of 2 to 4 volts for use in dentistry and surgery were made available. These miniature lamps had no bases, wires being used to connect them to the circuit. [Illustration: STANDARD EDISON LAMP, 1894. This lamp had a “treated” cellulose filament, permitting an efficiency of 3½ lumens per watt which has never been exceeded in a carbon lamp. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.]