The transmitter part of the single needle telegraph consists of a commutator operated by a shaft turned by a drop handle, which protrudes from the front of the instrument. As can be seen from the diagram, the commutator is placed in series with the line and the receiver and when at rest presents a short circuit, allowing signal to be received and passed on to other stations. When the drop handle is turned, the short circuit is removed and the battery connected to terminals Y and Z is connected in series with the line. The polarity of the battery is determined by which way the handle has been turned. By turning the handle to one side or the other and back to the centre again a series of pulses of current of varying polarity are sent to the line. Notice that the receiver is connected in series so that the operator has visual and audible feedback of the code being sent.

The receiver has an indicating needle on the faceplate which travels between two stops, which are designed to emit different notes when the needle hits them. The shaft holding the needle supports an armature which swings between two coils. The shaft and armature are split in the centre by a non-magnetic divider and a pair of horseshoe magnets polarise the armature as shown in the diagram. This method of operation is known as Spagnoletti's induced needle (or axle), named in 1869 after the famous Railway Telegraph Superintendent, Charles Ernest Spagnoletti (1832-1915). A current in the coils causes the needle to rotate. A positive current causes rotation in one direction, whilst a negative current causes rotation in the other direction.