6MAGNETO-ELECTRICITY.
19FORM OF A COMPOUND SOUND-WAVE IN AIR.
7MAGNETIC INDUCTION.
20HELMHOLTZ' ELECTRIC INTERRUPTOR.
8MAGNETO-ELECTRIC MACHINES.
21REISS' TELEPHONE.
9SECONDARY CURRENTS.
22GRAY'S TELEPHONES.
10THEORIES.
23BELL'S TELEPHONE.
11VELOCITY.
24FIRST FORM OF SPEAKING-TELEPHONE.
12CONSTITUTION OF A SINGLE SOUND-WAVE.
25THE AUTHOR'S TELEPHONE.
13PITCH.