The Collaborative International Dictionary of English v.0.48:

Thermophone \Ther"mo*phone\, n. [Thermo- + phone.]
   1. A portable form of telethermometer, using a telephone in
      connection with a differential thermometer.
      [Webster 1913 Suppl.]

   2. A telephone involving heat effects, as changes in
      temperature (hence in length) due to pulsations of the
      line current in a fine wire connected with the receiver
      diaphragm.
      [Webster 1913 Suppl.]