In this example I will be dividing an Arc but its the same principle for a straight line.
First identify the centre of the arc via its element properties. This time draw a line from the arcs centre point to one end of the arc.
With the straight line selected hit Array from the modify tab.You will need to select the button for radial array and the check-box saying Move To: Last.
Next we need to move the straight lines centre of rotation to the centre point of the arc. To do this simply click and hold the icon highlighted below and reposition.
Now you have set the rotation centre point you need to select a reference point to start the rotation. In this example select the start of the arc and rotate to the end of the arc.
You should now see something like the diagram below with a temporary dimension on the arrayed objects which is asking for the number of times the object is to be arrayed. You can input the required number directly into this temporary dim.
The results should be something like below:
The great thing about this is that if you change your mind as to how many times the object should be arrayed you can simply select one of the arrayed objects to access the temporary dim again and alter the number to suit.
This is by no means limited to lines, it can be applied to both detail and 3D families as well.
One thing to be aware of when using this method is that what you are actually doing is creating a group in the process. This is not a problem in itself however it is something you should be aware of . Having lots of groups and arrays within large projects can impact of the project's performance. This grouping behaviour is controlled within array command as highlighted below:
Having this tick box unchecked will still allow you to array the object as above however you will not be able to modify the array once its done.