As in resistance spot welding, the distribution of heat in the two sections being welded with the projection welding process must be reasonably uniform to obtain strong welds. The following factors affect heat balance: 1. Design and location of the projection, 2. Thickness of the sections, 3. Thermal and electrical conductivity of the sections, 4. Heating rate, and 5. Electrode alloy. The major portion of the heat develops in the projections during welding. Consequently, heat balance is generally easier to obtain in projection welding than in resistance spot welding; however, heat balance may be complicated when making multiple simultaneous projection welds. Uniform allocation of welding current and electrode force is necessary to obtain the even heating of all projections. Since the current paths through the projections are in parallel, any variation in resistance between the projections will cause the current to be distributed unequallyProjections must be designed to support the electrode force needed to obtain good electrical contact with the mating workpieces, and to collapse when heated. With multiple projections, slight variations in projection heights can affect heat balance. This may occur as a result of wear of the projection-forming punches. Heat balance in metals of dissimilar thickness is maintained by placing the projection in the thicker of the two workpieces. The size of the projection is based on the requirements for heating the thinner section. Likewise, to maintain heat balance in materials of dissimilar conductivity, the projection is located in the workpiece with the higher conductivity (lower resistivity). The electrode must be chosen on the basis of its composition because the electrode alloy determines the conductivity of the electrode, which also can affect heat balance.