| Year | 1921 |
| Date | 02-Jun |
| Volume | 44 |
| Issue | 1,049 |
| Page | 439 |
| Category | Electrical |
| Part | |
| Title | Further Notes on the "P.O." Resistance Box |
| Author | A.V.Ballhatchet |
| Notes | Diagram of dial switching attagements, arrangement when extending system to function with ratio arms of box |