diff --git a/04-One-Dimensional-Kalman-Filters.ipynb b/04-One-Dimensional-Kalman-Filters.ipynb
index 55666bd..1bba081 100644
--- a/04-One-Dimensional-Kalman-Filters.ipynb
+++ b/04-One-Dimensional-Kalman-Filters.ipynb
@@ -1639,9 +1639,10 @@
     "voltage_std = .13\n",
     "process_var = .05**2\n",
     "actual_voltage = 16.3\n",
+	"dt = 1. # time step in seconds\n",
     "\n",
     "x = (25, 1000) # initial state\n",
-    "process_model = (0., process_var)\n",
+    "process_model = (temp_change*dt, process_var)\n",
     "\n",
     "N = 50\n",
     "zs = [volt(actual_voltage, voltage_std) for i in range(N)]\n",