The a. c. welder with high efficiency due to the small stray load loss is also preferable in the standpoint of arc stability. The instantenuous value of the leakage flux of the transfcrmer is not zero at the instant of the zero value of the welding current due to the m. m. f. of the eddy current induced by the flux, so the secondary open voltage does not rises to its steady value in gtantenucusly by the damping action of the leakage flux above mentioned. The voltage recovery at open circuit due to arc vanishing lags only about 1/2000 second by a new designed welder which has a efficiency value of 88%, the time being appr-oximately one third of a old type welder, whose efficiency value is 70%. The difference of the arc stability of the above mentioned two welders is observable clearly by hand welding when we use some electrodes, but is not observable when we use some other electrodes. The arc voltage and current wave forms by a electrode of peer arc stability are shown in Fig.9, (a) and (b) being the case of the welder of quick and slow voltage recovery respectively. By a electrode of good arc stability, the arc voltage rises slowly after zero currnt intant (Fig.1), due to the good conduc tivity of the arc part, so the lagging of the voltage recovery of the welder does not affect on the arc stability.
Everybody recognizes that the welding residual stress is due to the external and internal constraints. As to special test piecer extenally constrained, some theoretical calculations have been made by Dr. Tanka, Dr. Naka, etc ... However, there is no report t discussing internal constraint. It is a wellknown fact that the internal constraint is due to temperature distributed nonuniformly but itsmechanism and rate have not been subjected to close examinations. In this report we dealt withsuch problems and have been led to the following conclusions. Inheret strain .aT (a. Coeft of expansion, T : temperature difference) do not always satisfy the condition of continuity. It goes without saying that the deformation which does not satisfy such a condition cannot occur. And this condition can be expressed by the compatibility equation of strain, which is the foundamental one in the theory-of elasticity. Therefore the rate of internal' constraint will be defined by the rate of aT which does nat satisfy that equation. From such considerations we can obtain some impartant characteristics of a family of isotherms. The differential equation for the temperature distribution of a welded plate is given by Rasenthal (J. A. W. S, . May 1941) as follows, ∂2T/∂ξ2+∂2T/∂y2=-v/k ∂T/∂ξ+1/k-∂T/∂t, whece ξ=x-vt, v : moving speed of electrode, k2 ;thermal diffusivity, t ; time. From the above equation, the constraining rate is expressed by F*=-v/k2 ∂T/∂ξ+1/k ∂T/∂t Apalysing the above, equation, we can get the following characteristics; (1) Constraing rate in the tangential directin of the isotherms is the greatest and that in the normal onl is the minimum. Tnen we know that the maximum principal stress'acts in the tangential direction and thh minimum one in the normal direcion of the curves. (2) Greater stress acts in the closer part of the isotherms than in the dispersed part. Examination of the equation of F will show the relations between the welding stress and the family of isotherms, welding conditions, etc.,