control register of DS1307

i made an adjustment on the adjust function of the RTClib.cpp.
i want to make the swq give a square pulse of 1Hz and i have added a wire.write with the byte 10010000 in hex.Is that the correct way? I used a polymeter and the sqw does not have any periodic voltage difference with gnd. I should have been 5volts i think but i get zero. I use adjust without checking if the clock is working(that means that the adjust function will run either way ).i use it at the setup().

void RTC_DS1307::adjust(const DateTime& dt) {

    Wire.beginTransmission(DS1307_ADDRESS);
    Wire.write(i);
    Wire.write(bin2bcd(dt.second()));
    Wire.write(bin2bcd(dt.minute()));
    Wire.write(bin2bcd(dt.hour()));
    Wire.write(bin2bcd(0));
    Wire.write(bin2bcd(dt.day()));
    Wire.write(bin2bcd(dt.month()));
    Wire.write(bin2bcd(dt.year() - 2000));
    Wire.write(0x90);//this is the hex of 10010000
 //   Wire.write(i);
    Wire.endTransmission();
}