Error LNK2001/LNK2009: unresolved external symbol

Question

I have a slight problem with my code and think I am missing out on something or don't understand.

Here is my code that basically reads the data from a .txt file and assigning the array to a float. In another .cpp, I want to have this array assigned to another variable but there error is always showing up:

TxtToArrayV2.cpp:

#include<iostream>
#include<fstream>
#include<string>
#include "TxtToArrayV2.h"

using namespace std;

// This program read the data from two files and extract the char data from it being after converted into a float number.

int main()
{
    ifstream foutRPM;
    ifstream foutTorque;

    foutRPM.open("rpmIRLBerlin.txt");
    foutTorque.open("torqueIRLBerlin.txt");

    char valueRPM[1000];
    char valueTorque[1000];

    while (!foutRPM.eof() & !foutTorque.eof())
    {
        foutRPM.getline(valueRPM, 10000);
        foutTorque.getline(valueTorque, 10000);

        // conversion from char to float: more info on https://www.programiz.com/cpp-programming/string-float-conversion 
        float RPM = stof(valueRPM);
        float Torque_Nm = stof(valueTorque);
        cout << RPM << "\n";
        cout << Torque_Nm << "\n";
    }

    foutRPM.close();
    foutTorque.close();

    system("pause");
    return 0;
}

TxtToArrayV2.h

#ifndef __TXT_TO_ARRAY_V2_H__
#define __TXT_TO_ARRAY_V2_H__

extern float RPM;
extern float Torque_Nm;


#endif

source.cpp:

#include "TxtToArrayV2.h"
#include<iostream>
#include<fstream>
#include<string>

void SomeFunctio()
{
    double lala = RPM;
}