A solution is obtained for the boundary of the magnetic field of two arbitrary line currents immersed in a plasma at uniform pressure. The linear dimensions of the boundary, which are determined for several sets of data, are proportional to the intensity of the line currents (assuming that their ratio is constant) and inversely proportional to the square root of the plasma pressure. The cusps over the boundary, which when the two line currents are equal and opposite are symmetrical with respect to the line currents, move towards the weaker current until they eventually disappear on the line joining the two currents. When the distance between the currents is such that when the cusps disappear the two line currents lie in the same cavity, some plasma gets trapped inside the cavity. When the cavities of two line currents of the same sign touch we get an abrupt instability; the plasma is suddenly pushed out by the surface currents and this results in the formation of a larger field cavity.