▁rule ▁for ▁construction ▁of ▁joint s ▁to ▁be ▁made ▁this ▁way ▁is ▁that ▁the ▁part ▁with ▁the ▁lower ▁mel ting ▁point ▁should ▁be ▁directly ▁accessible ▁for ▁the ▁beam . ▁ ▁Possible ▁problems ▁and ▁limitations ▁ ▁The ▁material ▁m elt ed ▁by ▁the ▁beam ▁shr inks ▁during ▁cool ing ▁after ▁solid ification , ▁which ▁may ▁have ▁unw anted ▁consequences ▁like ▁crack ing , ▁de formation ▁and ▁changes ▁of ▁shape , ▁depending ▁on ▁conditions . ▁ ▁The ▁but t ▁w eld ▁of ▁two ▁pl ates ▁results ▁in ▁b ending ▁of ▁the ▁w eld ment ▁because ▁more ▁material ▁has ▁been ▁m elt ed ▁at ▁the ▁head ▁than ▁at ▁the ▁root ▁of ▁the ▁w eld . ▁This ▁effect ▁is ▁of ▁course ▁not ▁as ▁substantial ▁as ▁in ▁arc ▁w eld ing . ▁ ▁Another ▁potential ▁danger ▁is ▁the ▁emer gence ▁of ▁cra cks ▁in ▁the ▁w eld . ▁If ▁both ▁parts ▁are ▁rig id , ▁the ▁shr ink age ▁of ▁the ▁w eld ▁produces ▁high ▁stress ▁in ▁the ▁w eld ▁which ▁may ▁lead ▁to ▁cra cks ▁if ▁the ▁material ▁is ▁brit t le ▁( even ▁if ▁only ▁after ▁rem el ting ▁by ▁w eld ing ). ▁The ▁consequences ▁of ▁w eld ▁contra ction ▁should ▁always ▁be ▁considered ▁when ▁construct ing ▁the ▁parts ▁to ▁be ▁w eld ed . ▁ ▁Electron - be am ▁w eld ing ▁equipment ▁ ▁Since ▁the ▁publication ▁of ▁the ▁first ▁practical ▁electron - be am ▁w eld ing ▁equipment ▁by ▁Ste iger wald ▁in ▁ 1 9 5 8 , ▁electron - be am