ClearAll["Global`*"];
$HistoryLength=2;
SetDirectory[NotebookDirectory[]];

FileNames[]

data1=<<"dataTrafo";
ListPlot[data1,AxesLabel->{"Ueff","Ief"},PlotRange->All]

ω=2.*Pi*50;
data2=data1/.{a_,b_}:>{((a*(Sqrt[2]))/ω),b};
ListPlot[data2,AxesLabel->{"ϕeff","Ief"},PlotRange->All];
ListPlot[data2,AxesLabel->{"ϕAmpl","Ief"},PlotRange->{Automatic,{0,1000}}]

mezfi=830;
datalin=Select[data2,#[[1]]<mezfi&];
(*test[{fi_,i_}]:=fi<mezfi;
datalin=Select[data2,test]*)

nahrlin=Fit[datalin,fi,fi];
a=D[nahrlin,fi];