ast ing ▁operation ▁was ▁determined ▁by ▁a ▁mechanical ▁clock work ▁timer ; ▁the ▁user ▁could ▁adjust ▁the ▁running ▁time ▁of ▁the ▁timer ▁to ▁determine ▁the ▁degree ▁of ▁to ast ing , ▁but ▁the ▁first ▁cycle ▁produced ▁less ▁to asted ▁to ast ▁than ▁subsequent ▁cycles ▁because ▁the ▁to aster ▁was ▁not ▁yet ▁war med ▁up . ▁To aster s ▁made ▁since ▁the ▁ 1 9 2 0 s ▁frequently ▁use ▁a ▁thermal ▁sensor , ▁such ▁as ▁a ▁b imet all ic ▁strip , ▁located ▁close ▁to ▁the ▁to ast . ▁This ▁allows ▁the ▁first ▁cycle ▁to ▁run ▁longer ▁than ▁subsequent ▁cycles . ▁The ▁thermal ▁device ▁is ▁also ▁slightly ▁respons ive ▁to ▁the ▁actual ▁temperature ▁of ▁the ▁to ast ▁itself . ▁Like ▁the ▁timer , ▁it ▁can ▁be ▁adjust ed ▁by ▁the ▁user ▁to ▁determine ▁the ▁don eness ▁of ▁the ▁to ast . ▁To aster s ▁may ▁also ▁use ▁an ▁electronic ▁control , ▁where ▁an ▁elect romagnet ▁holds ▁the ▁pop - up ▁mechanism ▁until ▁an ▁adjust able ▁time ▁delay ▁has ▁passed . ▁▁ ▁To aster s ▁may ▁also ▁be ▁used ▁to ▁to ast ▁other ▁food s ▁such ▁as ▁te ac akes , ▁to aster ▁pas try , ▁pot ato ▁w aff les ▁and ▁cr ump ets , ▁though ▁result ant ▁accum ulation ▁of ▁fat ▁and ▁sugar ▁inside ▁the ▁to aster ▁can ▁contribute ▁to ▁its ▁event ual ▁failure . ▁▁ ▁Among ▁pop - up ▁to aster s , ▁those ▁to ast ing ▁two ▁s lices ▁of ▁bread ▁are ▁more ▁purchased ▁than ▁those ▁which ▁can ▁to ast