Empty service call is not executed

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I implemented the basic empty service as proposed by the tutorial in my program:

bool callback(std_srvs::Empty::Request& request, std_srvs::Empty::Response& response) {
    return true;
}

ros::NodeHandle * privateNodeHandle; 

int main(int argc, char **argv) {
    ros::init(argc, argv, node_name);
    ros::NodeHandle n;
    privateNodeHandle = new ros::NodeHandle("~");
    privateNodeHandle->param("publishFrequency", publishFrequency, 1.0);
    privateNodeHandle->param("maximumWaterLevel", maximumWaterLevel, 1.0); 

    ros::Publisher water_level_pub = n.advertise<std_msgs::float64>(
            topic, 1000);
    ros::ServiceServer service = n.advertiseService("my_service", callback);
    ros::Rate loop_rate(publishFrequency); 

    tank = new WaterTank(maximumWaterLevel);
    int count = 0;
    while (ros::ok()) {
        std_msgs::Float64 msg;
        std::stringstream ss;
        ss << "Water level at " << count << " is " << tank->get_water_level();
        msg.data = tank->get_water_level();
        ROS_INFO("%s", ss.str().c_str());
        water_level_pub.publish(msg);
        loop_rate.sleep();
        ++count;
    }
    delete tank;
    delete privateNodeHandle;
    return 0;
}