Guarding the Telegraph, Chancellorsville
The Military Telegraph During the Civil War Volume 1

The regiment mentioned on 364 that guarded the line was the 20th Maine, see Official Reports Chancellorsville.
    In a letter to the Governor of Maine Chamberlain writes,
The Regiment was enabled to do good & important service during the fight, although we were not allowed to mingle with the rest of the Army. I had a midnight order from Gen. Butterfield to take possession of the signal wire from the Battlefield to Head Qrs. of the Army. This gave us enough to do, as the wire was tampered with & broken many times a night & communication was of the utmost importance. I was in the saddle all the nights inspecting every inch of the line. The Reg’t is in good health – never so free from sickness – small pox has entirely disappeared.*

Many thanks to John J. Pullen for finding this and listing it in his sources for his wonderful book the "Twentieth Maine." Though I am somewhat envious of his being able to interview people no longer alive, I will never envy how hard it was to find anything before the days of computers. Other places to view online:
books.google.com