This volume covers the period July-September 1877. Most of the notes and drawings are by Edison; others are by Charles Batchelor, James Adams, and John Kruesi. The material in this volume relates primarily to acoustic and speaking telegraphy. There are also a few drawings dealing with etheric force, the electric light, the phonograph, and the horse clipper. The volume consists of 189 numbered, unbound pages.

Missing pages: 41, 150, 180, 187. Numerous additional pages missing from this volume can be found in facsimile in the Telephone Interferences.

Several documents published in The Papers of Thomas A. Edison, Vol. 3 consist of pages from both Vol. 12 and the Telephone Interferences. For the sake of completeness, the appropriate pages from the Telephone Inteferences have been added to Vol. 12, and the images constituting each document have been arranged in their proper sequence:

1. Doc. 964: pages 7 (TI), 8 (Vol. 12)
2. Doc. 968: pages 20-21 (Vol. 12), 22 (TI)
3. Doc. 972: pages 26 (Vol. 12), 24 (TI), 25 (TI)
4. Doc. 1005: pages 104 (Vol. 12), 105 (TI)
5. Doc. 1013: pages 108 (Vol. 12); 115 (TI); 109, 112 (Vol. 12); 113 (TI); 114, 110 (Vol. 12); 111 (TI); 107 (Vol. 12)