calib_window_for_measurments *rectifierCurrent = new calib_window_for_measurments(rectifier_current());

В хедере:

#ifndef CALIB_WINDOW_FOR_MEASURMENTS_H
#define CALIB_WINDOW_FOR_MEASURMENTS_H

#include <QWidget>

namespace Ui {
class calib_window_for_measurments;
}

class calib_window_for_measurments : public QWidget
{
    Q_OBJECT

public:
    explicit calib_window_for_measurments(QWidget *parent = 0);
    ~calib_window_for_measurments();

protected:
    Ui::calib_window_for_measurments *ui;
};

class rectifier_current : public calib_window_for_measurments
{
    Q_OBJECT
public:
    explicit rectifier_current(QWidget *parent = 0) ;

};

class rectifier_voltage : public calib_window_for_measurments
{
    Q_OBJECT
public:
    explicit rectifier_voltage(QWidget *parent = 0) ;

};

#endif // CALIB_WINDOW_FOR_MEASURMENTS_H