The three ways in which the various wire connections of these machines are joined up, and which are enough for all ordinary purposes, are given below in paragraphs (_a_), (_b_), and (_c_). (_a_) When the machine is working _singly_ and revolving in the direction indicated in Fig. 166, the following connections are made:-- _M_ is connected with _B_, _M'_ " _B'_, _m_ " _Z_, _m'_ " _C_, and the leading wires of the lamp are connected with _C_ and with _Z_ as explained. (_b_) When working _singly_ and revolving in the direction indicated in Fig. 168:-- _M_ is connected to _B'_, _M'_ " _B_, _m_ " _Z_, _m'_ " _C_. Thus the only change necessary when the machine is to be driven in the opposite direction to that for which it is made, is to disconnect at _B_ the wire from _M_ to _B_ and at _B'_ the wire from _M'_ to _B'_, and to cross them. The machine will then be connected as above (_b_). (_c_) When working _two_ machines in parallel circuit, as in Fig. 174, they must be connected as follows (that on the left of the page being called the first machine, and that on the right the second machine):-- _C_ of first to _C_ of the second. _Z_ " _Z_ " _M_ " _B_ " _B_ " _M_ " _M'_ " _B'_ " _B'_ " _M'_ " and then connect _C_ and _Z_ of the second machine with the leading wires of the lamp. The connections _m_ to _Z_ and _m'_ to _C_ in each machine are the same as in cases (_a_) and (_b_). They do not require to be altered, and may therefore be left out of consideration in all three cases (_a_), (_b_), and (_c_). The whole of the connections here indicated can be quickly made by means of a cross-bar commutator or switch, which is supplied with the machines in cases where such changes are likely to be required frequently. This is usually attached to a wall, leading wires being taken to it from the dynamo machines separately, and others from the switch being led to the electric lamps. The leading wires from machine to lamp should, whenever possible, be kept _separate_, to prevent them rubbing together and making contact. A distance of two inches is quite sufficient to prevent accidents of any kind. When the leading wires are erected in places where they are likely to rub and chafe against hard substances, it is advisable to enclose each wire separately in india-rubber tubing at all the points where they are likely to be rubbed. This becomes very important on board ship, where everything is in motion, and special care is in consequence required.