For d.c. track circuits, four types of cells have been used to a greater or less extent, the gravity cell; Lalande (soda) cell; storage cell and dry cell. The gravity cell has a voltage of about 0.8 or 0.9 volts, the resistance varying with the manner in which the cell is maintained and averaging about 3 ohms. It will remain active for long periods on closed circuits without appreciable polarization. Because of this high internal resistance usually no external resistance is necessary to be connected between it and the rail of the track. The e.m.f. of the Lalande (soda) cell may vary from about 0.67 volts to 0.88 volts while the internal resistance will range between 0.019 ohm to 0.4 ohm. Because of the low internal resistance of these cells it is necessary to use an external resistance of the proper value between the cell and the rail. The storage cell is made in various capacities and a fully charged cell on open circuit has a voltage of approximately 2.1 volts which, when placed on discharge, becomes approximately 2 volts and drops to about 1.8 volts when completely discharged. The voltage in this type of cell varies with the density of the electrolyte and to a certain extent with temperature. It has practically a negligible internal resistance and it is also necessary to use an external resistance in the leads between the cell and the track to prevent a flow of excessive current when a train occupies the track. The dry cell is used only in emergency cases or occasionally for open circuit track circuits of 2 or 3 rail lengths, which are sometimes used as annunciator starts to announce the approach of a train to a tower-man. It is designed primarily for open circuit work and will polarize when current beyond a certain figure is drawn continuously from it. The Track Relay The track relay is a development of the instrument of the same name used in telegraph service. It consists of an electro-magnet of the horseshoe type with a pivoted armature, carrying one or more fingers for making or breaking electric circuits for the control of signal apparatus. Track relays with resistances of 2 and 4 ohms are usually employed. From experience with two-ohm relays on the L. & N., covering a great many of them on all kinds of circuits, the following conclusions are reached: The two-ohm relay is more suitable for general use on track circuits than the four-ohm, provided not less than the R.S.A. recommended limiting resistance is used between the battery and track. The two-ohm relay will operate satisfactorily where the four-ohm will not on bad track circuits, and with considerably less current consumption. The two-ohm relay will operate equally as well on good track circuits of average length as the four-ohm, there being little difference in current consumption on this class of circuit. Under the same conditions longer track circuits may be operated with the two-ohm relay.