From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Summary of the physical properties for different grades of carbon products from British Carbons.
| Identifier | ExFiles\Box 166\1\ img010 | |
| Date | 2nd November 1922 guessed | |
| TEL. 5225 CODES: A.B.C. 5TH EDITION. LIEBER'S. DIRECTORS: A.{Mr Adams} SOUTHERN. H.{Arthur M. Hanbury - Head Complaints} SAMPSON. B. HOLBREY. British Carbons LIMITED. MANUFACTURERS OF CARBON BRUSHES FOR MOTORS & DYNAMOS. CAR DYNAMOS & STARTER BRUSHES. MAGNETO CARBONS. PROGRESS WORKS. CROWTHER STREET. BRADFORD, YORKS. SUMMARY OF PHYSICAL PROPERTIES OF GRADES. [Table] GRADE | CARRYING CAPACITY Amps per Sq. inch | PERIPHERAL SPEED Feet per Minute | RESISTANCE Ohms per Cu.inch | HARDNESS (Scleroscope) | CONTACT DROP G.20 | 58 | 4000 | .00100 | 48 | LOW G.7. | 65 | 12000 | .00080 | 16 | LOW M.75. | 97 | 6000 | .00002 | 21 | VERY LOW M.85 | 130 | 5000 | .0000050 | 24 | VERY LOW | ||