Fig. 4a.
The signal mechanism used on the Robinson signal at Elm street was of the electro-mechanical type.
Figure 4 is a half tone of the identical signal mechanism in operation there when the Emperor of Brazil examined the system with Mr. Robinson, on June 14, 1876.
It is pointed out that the above signal mechanism, Fig. 4, shows a battery or pole changing attachment which is more clearly shown in Fig. 4a, reproduced from Robinson's British patent No. 3479 of August 29, 1879.
In this device the movement of the cam i2 not only changes the battery but changes the polarity through the magnet M2, which may be placed anywhere and used for any purpose.
A special device for the same purpose was used not only in connection with the relay on the West Somerville signal, but on many others installed by Mr. Robinson.
This battery and pole changing device is more fully described in Robinson's U.S. patent, August 25, 1874, No. 154,520, Automatic Commutator; Application filed July 18, 1873.
The following extract therefrom, and claim, may be interesting:
"It will be observed also, that while the actual change of battery may be caused to take place when the magnet attracts its armature, yet I prefer to arrange it so that no change of connections shall take place when the armature is attracted, the actual change taking place only during the reverse movement of the armature, caused by the retractile force of the spring. Furthermore, when desired, the batteries may be so connected in circuit that reverse currents shall be passed through the magnets every time the batteries are changed."