Warum kann ich am ESP das Relais nur einmal schalten?

International Deutsch
21 
String off = "http://192.168.1.143/relay_off";
String on  = "http://192.168.1.143/relay_on";
#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
HTTPClient sender;
WiFiClient wifiClient;

const char* ssid       = "xyz";
const char* password   = "1234";

void setup() {
  Serial.begin(115200);
  Serial.println();
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(200);
    Serial.print(".");
  }
  Serial.println("\rVerbunden!");
  if (sender.begin(wifiClient, on))  {
    Serial.println("Befehl = " + on);
    int httpCode = sender.GET();
    if (httpCode > 0) {
      if (httpCode == HTTP_CODE_OK)Serial.println(sender.getString());
    } else      Serial.printf("HTTP - Error: ", sender.errorToString(httpCode).c_str());
  } else     Serial.printf("HTTP - Verbindung konnte nicht hergestellt werden!");
}

void loop(){
  sender.setURL(off);
  delay(1999);
  sender.setURL(on);
  delay(1999);
}
22

Danke!