The Collaborative International Dictionary of English v.0.48:

Diagometer \Di`a*gom"e*ter\, n. [Gr. ? to transmit + -meter.]
   A sort of electroscope, invented by Rousseau, in which the
   dry pile is employed to measure the amount of electricity
   transmitted by different bodies, or to determine their
   conducting power. --Nichol.
   [1913 Webster]