4.
某同学用图甲所示电路测量电流表\(G\)\({\,\!}_{1}\)的内阻\(r\)\({\,\!}_{1}\)\(.\)供选择的仪器如下:待测电流表\(G\)\({\,\!}_{1}\)\((0~5mA\),内阻约\(200\)\(Ω \)\()\);电流表\(G\)\({\,\!}_{2}\)\((0~10mA\),内阻约\(100\)\(Ω \)\()\);定值电阻\(R\)\({\,\!}_{1}\)\((100\)\(Ω \)\()\);定值电阻\(R\)\({\,\!}_{2}\)\((200\)\(Ω \)\()\);滑动变阻器\(R\)\({\,\!}_{3}\)\((0~1000\)\(Ω \)\()\);滑动变阻器\(R\)\({\,\!}_{4}\)\((0~100\)\(Ω \)\()\);电源\(E(\)电动势\(6V\),内阻可不计\()\);开关\(S\)及导线若干.
\((1)\)实验中要求电流表\(G\)\({\,\!}_{1}\)能达到满偏,则定值电阻应选_____\((\)填“\(R\)\({\,\!}_{1}\)”或“\(R\)\({\,\!}_{2}\)”\()\),滑动变阻器应选________\((\)填“\(R\)\({\,\!}_{3}\)”或“\(R\)\({\,\!}_{4}\)”\()\),按照电路图,闭合开关\(S\)前应将滑动触头移至_____端\((\)填“\(a\)”或“\(b\)”\()\).
\((2)\)在图乙所示实物图中,已正确连接部分导线,请根据图甲电路完成剩余部分的连接.
\((3)\)实验步骤如下:按电路图连接电路,将滑动变阻器的滑动触头移至一端;闭合开关\(S\),移动滑动触头至某一位置,记录\(G_{1}\)、\(G_{2}\)的读数\(I_{1}\)、\(I_{2}\);多次移动滑动触头,记录相应的\(G_{1}\)、\(G_{2}\)的读数\(I_{1}\)、\(I_{2}\);以\(I_{1}\)为纵坐标,\(I_{2}\)为横坐标,作出相应图线,如图丙所示\(.\)若图中直线的斜率为\(k\),请用题中所给符号写出待测电流表内阻的表达式\(r_{1}=\)___________.